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.

spec/services/site_settings_spec.rb

@@ -11,13 +11,13 @@ describe SiteSettingsTask do
   describe 'export' do
     it 'creates a hash of all site settings' do
       sso_url = "https://somewhere.over.com"
-      SiteSetting.sso_url = sso_url
-      SiteSetting.enable_sso = true
+      SiteSetting.discourse_connect_url = sso_url
+      SiteSetting.enable_discourse_connect = true
       hash = SiteSettingsTask.export_to_hash
 
       expect(hash).to eq(
-        "enable_sso" => "true",
-        "sso_url" => sso_url
+        "enable_discourse_connect" => "true",
+        "discourse_connect_url" => sso_url
       )
     end
   end

spec/services/user_updater_spec.rb

@@ -267,9 +267,9 @@ describe UserUpdater do
 
     context 'when sso overrides bio' do
       it 'does not change bio' do
-        SiteSetting.sso_url = "https://www.example.com/sso"
-        SiteSetting.enable_sso = true
-        SiteSetting.sso_overrides_bio = true
+        SiteSetting.discourse_connect_url = "https://www.example.com/sso"
+        SiteSetting.enable_discourse_connect = true
+        SiteSetting.discourse_connect_overrides_bio = true
 
         user = Fabricate(:user)
         updater = UserUpdater.new(acting_user, user)
@@ -283,9 +283,9 @@ describe UserUpdater do
 
     context 'when sso overrides location' do
       it 'does not change location' do
-        SiteSetting.sso_url = "https://www.example.com/sso"
-        SiteSetting.enable_sso = true
-        SiteSetting.sso_overrides_location = true
+        SiteSetting.discourse_connect_url = "https://www.example.com/sso"
+        SiteSetting.enable_discourse_connect = true
+        SiteSetting.discourse_connect_overrides_location = true
 
         user = Fabricate(:user)
         updater = UserUpdater.new(acting_user, user)
@@ -299,9 +299,9 @@ describe UserUpdater do
 
     context 'when sso overrides website' do
       it 'does not change website' do
-        SiteSetting.sso_url = "https://www.example.com/sso"
-        SiteSetting.enable_sso = true
-        SiteSetting.sso_overrides_website = true
+        SiteSetting.discourse_connect_url = "https://www.example.com/sso"
+        SiteSetting.enable_discourse_connect = true
+        SiteSetting.discourse_connect_overrides_website = true
 
         user = Fabricate(:user)
         updater = UserUpdater.new(acting_user, user)