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.
Also adds idle time and keepalive parameters because how this is
configured has changed in the new package version. The values are those
that seems like might already be default, if keep-alives were enabled,
which is not obvious from the doc comments.
Also, Go 1.19 gofmt reformatting of comments.
