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/script/import_scripts/base_spec.rb

@@ -49,26 +49,12 @@ describe ImportScripts::Base do
 
   it "creates bookmarks, posts, and users" do
     MockSpecImporter.new(import_data).perform
-    expect(Bookmark.count).to eq(5)
+    expect(Bookmark.where(bookmarkable_type: "Post").count).to eq(5)
     expect(Post.count).to eq(5)
     expect(User.where('id > 0').count).to eq(1)
     expect(SiteSetting.purge_unactivated_users_grace_period_days).to eq(60)
   end
 
-  context "when polymorphic bookmarks are enabled" do
-    before do
-      SiteSetting.use_polymorphic_bookmarks = true
-    end
-
-    it "creates bookmarks, posts, and users" do
-      MockSpecImporter.new(import_data).perform
-      expect(Bookmark.where(bookmarkable_type: "Post").count).to eq(5)
-      expect(Post.count).to eq(5)
-      expect(User.where('id > 0').count).to eq(1)
-      expect(SiteSetting.purge_unactivated_users_grace_period_days).to eq(60)
-    end
-  end
-
   it "does not change purge unactivated users setting if disabled" do
     SiteSetting.purge_unactivated_users_grace_period_days = 0
     MockSpecImporter.new(import_data).perform