FIX: Store Reviewable's force_review as a boolean. (#11219)

* FIX: Store Reviewable's force_review as a boolean.

Using the `force_review` flag raises the score to hit the minimum visibility threshold. This strategy turned out to be ineffective on sites with a high number of flags, where these values could rapidly fluctuate.

This change adds a `force_review` column on the reviewables table and modifies the `Reviewable#list_for` method to show these items when passing the `status: :pending` option, even if the score is not high enough. ReviewableQueuedPosts and ReviewableUsers are always created using this option.

db/migrate/20201112142419_add_reviewables_force_review.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddReviewablesForceReview < ActiveRecord::Migration[6.0]
+  def change
+    add_column :reviewables, :force_review, :boolean, default: false, null: false
+  end
+end