jf_api now caches the list of users, only fetching new data if the cache
is more than 30 minutes old. Also remove the pointless tempJF with an
instance named auth_jf since jf_api can now handle multiple
authentications.
The new jellyfin_login and admin_only allow anyone use their username and
password from jellyfin to login to the admin page, and restrict this to jellyfin
admins only, respectively.