imapserver: implement PREVIEW extension (RFC 8970), and store previews in message database

We were already generating previews of plain text parts for the webmail
interface, but we didn't store them, so were generating the previews each time
messages were listed.

Now we store previews in the database for faster handling. And we also generate
previews for html parts if needed. We use the first part that has textual
content.

For IMAP, the previews can be requested by an IMAP client. When we get the
"LAZY" variant, which doesn't require us to generate a preview, we generate it
anyway, because it should be fast enough. So don't make clients first ask for
"PREVIEW (LAZY)" and then again a request for "PREVIEW".

We now also generate a preview when a message is added to the account. Except
for imports. It would slow us down, the previews aren't urgent, and they will
be generated on-demand at first-request.

webaccount/import.go

@@ -551,6 +551,7 @@ func importMessages(ctx context.Context, log mlog.Log, token string, acc *store.
 			SkipThreads:         true,
 			SkipUpdateDiskUsage: true,
 			SkipCheckQuota:      true,
+			SkipPreview:         true,
 		}
 		if err := acc.MessageAdd(log, tx, mb, m, f, opts); err != nil {
 			problemf("delivering message %s: %s (continuing)", pos, err)