This splits the Login and Authentication admin page into multiple
sections: Settings, social logins, DiscourseConnect, OAuth2.0, OIDC.
Internal ticket `t/161648`
This PR also adds a new extension point for plugins that want to extend
this area of the app.
```
register_site_setting_area("oauth2")
register_admin_config_login_route("oauth2")
```
Adding this to a plugin allows it to extend both the setting areas
(existing functionality) and add a tab to this admin settings screen via
`register_admin_config_login_route`.