Merge new setup wizard
This is much more up-to-date than the previous setup page, with a new
design and previously missing/new settings. Currently only available in
english (hopefully that changes soon).
also fixes conflict in _post.
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.