FEATURE: Rename 'Discourse SSO' to DiscourseConnect (#11978)

The 'Discourse SSO' protocol is being rebranded to DiscourseConnect. This should help to reduce confusion when 'SSO' is used in the generic sense.

This commit aims to:
- Rename `sso_` site settings. DiscourseConnect specific ones are prefixed `discourse_connect_`. Generic settings are prefixed `auth_`
- Add (server-side-only) backwards compatibility for the old setting names, with deprecation notices
- Copy `site_settings` database records to the new names
- Rename relevant translation keys
- Update relevant translations

This commit does **not** aim to:
- Rename any Ruby classes or methods. This might be done in a future commit
- Change any URLs. This would break existing integrations
- Make any changes to the protocol. This would break existing integrations
- Change any functionality. Further normalization across DiscourseConnect and other auth methods will be done separately

The risks are:
- There is no backwards compatibility for site settings on the client-side. Accessing auth-related site settings in Javascript is fairly rare, and an error on the client side would not be security-critical.
- If a plugin is monkey-patching parts of the auth process, changes to locale keys could cause broken error messages. This should also be unlikely. The old site setting names remain functional, so security-related overrides will remain working.

A follow-up commit will be made with a post-deploy migration to delete the old `site_settings` rows.

app/controllers/application_controller.rb

@@ -739,11 +739,11 @@ class ApplicationController < ActionController::Base
   def redirect_to_login
     dont_cache_page
 
-    if SiteSetting.external_auth_immediately && SiteSetting.enable_sso?
+    if SiteSetting.auth_immediately && SiteSetting.enable_discourse_connect?
       # save original URL in a session so we can redirect after login
       session[:destination_url] = destination_url
       redirect_to path('/session/sso')
-    elsif SiteSetting.external_auth_immediately && !SiteSetting.enable_local_logins && Discourse.enabled_authenticators.length == 1 && !cookies[:authentication_data]
+    elsif SiteSetting.auth_immediately && !SiteSetting.enable_local_logins && Discourse.enabled_authenticators.length == 1 && !cookies[:authentication_data]
       # Only one authentication provider, direct straight to it.
       # If authentication_data is present, then we are halfway though registration. Don't redirect offsite
       cookies[:destination_url] = destination_url