lib/api: Save session & CSRF tokens to database, add option to stay logged in (fixes #9151) (#9284)

This adds a "token manager" which handles storing and checking expired
tokens, used for both sessions and CSRF tokens. It removes the old,
corresponding functionality for CSRFs which saved things in a file. The
result is less crap in the state directory, and active login sessions
now survive a Syncthing restart (this really annoyed me).

It also adds a boolean on login to create a longer-lived session cookie,
which is now possible and useful. Thus we can remain logged in over
browser restarts, which was also annoying... :)

<img width="1001" alt="Screenshot 2023-12-12 at 09 56 34"
src="https://github.com/syncthing/syncthing/assets/125426/55cb20c8-78fc-453e-825d-655b94c8623b">

Best viewed with whitespace-insensitive diff, as a bunch of the auth
functions became methods instead of closures which changed indentation.

cmd/syncthing/main.go

@@ -862,6 +862,7 @@ func cleanConfigDirectory() {
 		"backup-of-v0.8":     30 * 24 * time.Hour, // these neither
 		"tmp-index-sorter.*": time.Minute,         // these should never exist on startup
 		"support-bundle-*":   30 * 24 * time.Hour, // keep old support bundle zip or folder for a month
+		"csrftokens.txt":     0,                   // deprecated, remove immediately
 	}
 
 	for pat, dur := range patterns {