add ability to include custom css & js in web interface (webmail, webaccount, webadmin), and use css variables in webmail for easier customization

if files {webmail,webaccount,webadmin}.{css,js} exist in the configdir (where
the mox.conf file lives), their contents are included in the web apps.

the webmail now uses css variables, mostly for colors. so you can write a
custom webmail.css that changes the variables, e.g.:

	:root {
		--color: blue
	}

you can also look at css class names and override their styles.

in the future, we may want to make some css variables configurable in the
per-user settings in the webmail. should reduce the number of variables first.

any custom javascript is loaded first. if it defines a global function
"moxBeforeDisplay", that is called each time a page loads (after
authentication) with the DOM element of the page content as parameter. the
webmail is a single persistent page. this can be used to make some changes to
the DOM, e.g. inserting some elements. we'll have to see how well this works in
practice. perhaps some patterns emerge (e.g. adding a logo), and we can make
those use-cases easier to achieve.

helps partially with issue #114, and based on questions from laura-lilly on
matrix.

webmail/webmail.go

@@ -12,6 +12,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"log/slog"
 	"mime"
 	"net/http"
@@ -21,6 +22,7 @@ import (
 	"runtime/debug"
 	"strconv"
 	"strings"
+	"time"
 
 	_ "embed"
 
@@ -147,27 +149,62 @@ func xdbread(ctx context.Context, acc *store.Account, fn func(tx *bstore.Tx)) {
 }
 
 var webmailFile = &mox.WebappFile{
-	HTML:     webmailHTML,
-	JS:       webmailJS,
-	HTMLPath: filepath.FromSlash("webmail/webmail.html"),
-	JSPath:   filepath.FromSlash("webmail/webmail.js"),
+	HTML:       webmailHTML,
+	JS:         webmailJS,
+	HTMLPath:   filepath.FromSlash("webmail/webmail.html"),
+	JSPath:     filepath.FromSlash("webmail/webmail.js"),
+	CustomStem: "webmail",
 }
 
-// Serve content, either from a file, or return the fallback data. Caller
-// should already have set the content-type. We use this to return a file from
-// the local file system (during development), or embedded in the binary (when
-// deployed).
-func serveContentFallback(log mlog.Log, w http.ResponseWriter, r *http.Request, path string, fallback []byte) {
+func customization() (css, js []byte, err error) {
+	if css, err = os.ReadFile(mox.ConfigDirPath("webmail.css")); err != nil && !errors.Is(err, fs.ErrNotExist) {
+		return nil, nil, err
+	}
+	if js, err = os.ReadFile(mox.ConfigDirPath("webmail.js")); err != nil && !errors.Is(err, fs.ErrNotExist) {
+		return nil, nil, err
+	}
+	css = append([]byte("/* Custom CSS by admin from $configdir/webmail.css: */\n"), css...)
+	js = append([]byte("// Custom JS by admin from $configdir/webmail.js:\n"), js...)
+	js = append(js, '\n')
+	return css, js, nil
+}
+
+// Serve HTML content, either from a file, or return the fallback data. If
+// customize is set, css/js is inserted if configured. Caller should already have
+// set the content-type. We use this to return a file from the local file system
+// (during development), or embedded in the binary (when deployed).
+func serveContentFallback(log mlog.Log, w http.ResponseWriter, r *http.Request, path string, fallback []byte, customize bool) {
+	serve := func(mtime time.Time, rd io.ReadSeeker) {
+		if customize {
+			buf, err := io.ReadAll(rd)
+			if err != nil {
+				log.Errorx("reading content to customize", err)
+				http.Error(w, "500 - internal server error - reading content to customize", http.StatusInternalServerError)
+				return
+			}
+			customCSS, customJS, err := customization()
+			if err != nil {
+				log.Errorx("reading customizations", err)
+				http.Error(w, "500 - internal server error - reading customizations", http.StatusInternalServerError)
+				return
+			}
+			buf = bytes.Replace(buf, []byte("/* css placeholder */"), customCSS, 1)
+			buf = bytes.Replace(buf, []byte("/* js placeholder */"), customJS, 1)
+			rd = bytes.NewReader(buf)
+		}
+		http.ServeContent(w, r, "", mtime, rd)
+	}
+
 	f, err := os.Open(path)
 	if err == nil {
 		defer f.Close()
 		st, err := f.Stat()
 		if err == nil {
-			http.ServeContent(w, r, "", st.ModTime(), f)
+			serve(st.ModTime(), f)
 			return
 		}
 	}
-	http.ServeContent(w, r, "", mox.FallbackMtime(log), bytes.NewReader(fallback))
+	serve(mox.FallbackMtime(log), bytes.NewReader(fallback))
 }
 
 func init() {
@@ -261,7 +298,7 @@ func handle(apiHandler http.Handler, isForwarded bool, accountPath string, w htt
 		}
 
 		w.Header().Set("Content-Type", "application/javascript; charset=utf-8")
-		serveContentFallback(log, w, r, path, fallback)
+		serveContentFallback(log, w, r, path, fallback, false)
 		return
 	}
 
@@ -621,7 +658,7 @@ func handle(apiHandler http.Handler, isForwarded bool, accountPath string, w htt
 
 		path := filepath.FromSlash("webmail/msg.html")
 		fallback := webmailmsgHTML
-		serveContentFallback(log, w, r, path, fallback)
+		serveContentFallback(log, w, r, path, fallback, true)
 
 	case len(t) == 2 && t[1] == "parsedmessage.js":
 		// Used by msg.html, for the msg* endpoints, for the data needed to show all data
@@ -689,7 +726,7 @@ func handle(apiHandler http.Handler, isForwarded bool, accountPath string, w htt
 		// from disk.
 		path := filepath.FromSlash("webmail/text.html")
 		fallback := webmailtextHTML
-		serveContentFallback(log, w, r, path, fallback)
+		serveContentFallback(log, w, r, path, fallback, true)
 
 	case len(t) == 2 && (t[1] == "html" || t[1] == "htmlexternal"):
 		// Returns the first HTML part, with "cid:" URIs replaced with an inlined datauri