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/requests/application_controller_spec.rb

@@ -22,8 +22,8 @@ RSpec.describe ApplicationController do
     end
 
     it "should redirect to SSO if enabled" do
-      SiteSetting.sso_url = 'http://someurl.com'
-      SiteSetting.enable_sso = true
+      SiteSetting.discourse_connect_url = 'http://someurl.com'
+      SiteSetting.enable_discourse_connect = true
       get "/"
       expect(response).to redirect_to("/session/sso")
     end
@@ -45,17 +45,17 @@ RSpec.describe ApplicationController do
       expect(response).to redirect_to("/login")
     end
 
-    it "should not redirect to SSO when external_auth_immediately is disabled" do
-      SiteSetting.external_auth_immediately = false
-      SiteSetting.sso_url = 'http://someurl.com'
-      SiteSetting.enable_sso = true
+    it "should not redirect to SSO when auth_immediately is disabled" do
+      SiteSetting.auth_immediately = false
+      SiteSetting.discourse_connect_url = 'http://someurl.com'
+      SiteSetting.enable_discourse_connect = true
 
       get "/"
       expect(response).to redirect_to("/login")
     end
 
-    it "should not redirect to authenticator when external_auth_immediately is disabled" do
-      SiteSetting.external_auth_immediately = false
+    it "should not redirect to authenticator when auth_immediately is disabled" do
+      SiteSetting.auth_immediately = false
       SiteSetting.enable_google_oauth2_logins = true
       SiteSetting.enable_local_logins = false