FEATURE: Promote polymorphic bookmarks to default and migrate (#16729)

This commit migrates all bookmarks to be polymorphic (using the
bookmarkable_id and bookmarkable_type) columns. It also deletes
all the old code guarded behind the use_polymorphic_bookmarks setting
and changes that setting to true for all sites and by default for
the sake of plugins.

No data is deleted in the migrations, the old post_id and for_topic
columns for bookmarks will be dropped later on.

spec/requests/admin/users_controller_spec.rb

@@ -308,7 +308,8 @@ RSpec.describe Admin::UsersController do
     it "also prevents use of any api keys" do
       api_key = Fabricate(:api_key, user: user)
       post "/bookmarks.json", params: {
-        post_id: Fabricate(:post).id
+        bookmarkable_id: Fabricate(:post).id,
+        bookmarkable_type: "Post"
       }, headers: { HTTP_API_KEY: api_key.key }
       expect(response.status).to eq(200)