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.

lib/validators/enable_invite_only_validator.rb

@@ -7,10 +7,10 @@ class EnableInviteOnlyValidator
 
   def valid_value?(val)
     return true if val == 'f'
-    !SiteSetting.enable_sso?
+    !SiteSetting.enable_discourse_connect?
   end
 
   def error_message
-    I18n.t('site_settings.errors.sso_invite_only')
+    I18n.t('site_settings.errors.discourse_connect_invite_only')
   end
 end

lib/validators/enable_sso_validator.rb

@@ -7,12 +7,12 @@ class EnableSsoValidator
 
   def valid_value?(val)
     return true if val == 'f'
-    return false if SiteSetting.sso_url.blank? || SiteSetting.invite_only?
+    return false if SiteSetting.discourse_connect_url.blank? || SiteSetting.invite_only?
     true
   end
 
   def error_message
-    return I18n.t('site_settings.errors.sso_url_is_empty') if SiteSetting.sso_url.blank?
-    return I18n.t('site_settings.errors.sso_invite_only') if SiteSetting.invite_only?
+    return I18n.t('site_settings.errors.discourse_connect_url_is_empty') if SiteSetting.discourse_connect_url.blank?
+    return I18n.t('site_settings.errors.discourse_connect_invite_only') if SiteSetting.invite_only?
   end
 end