add funtionality to import zip/tgz with maildirs/mboxes to account page

so users can easily take their email out of somewhere else, and import it into mox.

this goes a little way to give feedback as the import progresses: upload
progress is shown (surprisingly, browsers aren't doing this...), imported
mailboxes/messages are counted (batched) and import issues/warnings are
displayed, all sent over an SSE connection. an import token is stored in
sessionstorage. if you reload the page (e.g. after a connection error), the
browser will reconnect to the running import and show its progress again. and
you can just abort the import before it is finished and committed, and nothing
will have changed.

this also imports flags/keywords from mbox files.

testdata/httpaccount/domains.conf

@@ -0,0 +1,26 @@
+Domains:
+	mox.example: nil
+Accounts:
+	mjl:
+		Domain: mox.example
+		Destinations:
+			mjl:
+				Mailbox: Inbox
+				Rulesets:
+					-
+						HeadersRegexp:
+								subject: test
+						Mailbox: Test
+					-
+						HeadersRegexp:
+								subject: .*
+						Mailbox: Catchall
+			other:
+				Mailbox: Other
+		JunkFilter:
+			Threshold: 0.950000
+			Params:
+				Twograms: true
+				MaxPower: 0.100000
+				TopWords: 10
+				IgnoreWords: 0.100000

testdata/httpaccount/mox.conf

@@ -0,0 +1,8 @@
+DataDir: data
+LogLevel: trace
+Hostname: mox.example
+Postmaster:
+	Account: mjl
+	Mailbox: postmaster
+Listeners:
+	local: nil

testdata/importtest.maildir/dovecot-keywords

@@ -0,0 +1,3 @@
+0 $Junk
+1 $Forwarded
+2 $NotJunk

testdata/importtest.mbox

@@ -9,10 +9,12 @@ Date: Wed, 10 Nov 2021 23:47:13 +0100
 Message-ID: <12312312-f95c-09ec-97c6-94d124f0932d@mox.test>
 MIME-Version: 1.0
 Status: RO
+X-Status: AFTD
+X-Keywords: $NotJunk,$Forwarded
 Content-Length: 15
 Lines: 3
 
-test
+>From testing
 test2
 end