FEATURE: Use full page redirection for all external auth methods (#8092)

Using popups is becoming increasingly rare. Full page redirects are already used on mobile, and for some providers. This commit removes all logic related to popup authentication, leaving only the full page redirect method.

For more info, see https://meta.discourse.org/t/do-we-need-popups-for-login/127988

spec/components/plugin/instance_spec.rb

@@ -161,20 +161,26 @@ describe Plugin::Instance do
   end
 
   it 'patches the enabled? function for auth_providers if not defined' do
+    SimpleAuthenticator = Class.new(Auth::Authenticator) do
+      def name
+        "my_authenticator"
+      end
+    end
+
     plugin = Plugin::Instance.new
 
     # lets piggy back on another boolean setting, so we don't dirty our SiteSetting object
     SiteSetting.enable_badges = false
 
     # No enabled_site_setting
-    authenticator = Auth::Authenticator.new
+    authenticator = SimpleAuthenticator.new
     plugin.auth_provider(authenticator: authenticator)
     plugin.notify_before_auth
     expect(authenticator.enabled?).to eq(true)
 
     # With enabled site setting
     plugin = Plugin::Instance.new
-    authenticator = Auth::Authenticator.new
+    authenticator = SimpleAuthenticator.new
     plugin.auth_provider(enabled_setting: 'enable_badges', authenticator: authenticator)
     plugin.notify_before_auth
     expect(authenticator.enabled?).to eq(false)
@@ -183,7 +189,7 @@ describe Plugin::Instance do
     plugin = Plugin::Instance.new
 
     SiteSetting.enable_badges = true
-    authenticator = Class.new(Auth::Authenticator) do
+    authenticator = Class.new(SimpleAuthenticator) do
       def enabled?
         false
       end