FEATURE: Gracefully handle unhandled reviewables (#31118)

Plugins like for example AI or Akismet create reviewable items. When the
plugin is disabled, then we cannot properly handle those items.

In that situation, we should display warnings about unhandled types.
Instruct admin to reenable plugins. In addition, we should allow the
admin to delete all pending reviews from disabled plugins.

app/controllers/admin/unknown_reviewables_controller.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class Admin::UnknownReviewablesController < Admin::AdminController
+  def destroy
+    Reviewable.destroy_unknown_types!
+    render json: success_json
+  end
+end

app/controllers/reviewables_controller.rb

@@ -74,6 +74,7 @@ class ReviewablesController < ApplicationController
           total_rows_reviewables: total_rows,
           types: meta_types,
           reviewable_types: Reviewable.types,
+          unknown_reviewable_types: Reviewable.unknown_types,
           score_types:
             ReviewableScore
               .types