Also changed some section names to use "messages" instead of "emails".
Since I haven't added Discord/Telegram/Matrix bot setup, I mentioned you
can do these later and linked to the setup guides. Sections related to
email mostly now depend on [messages]/enabled now too. Set password via
link has been added to password resets.
Also make store each setting as classes in a settings object, to make it
easier to serialize on submitting. Also, added
"substitute_jellyfin_strings", "no_username" and welcome_email.