FEATURE: Flag count in post menu

This change shows a notification number besides the flag icon in the
post menu if there is reviewable content associated with the post.
Additionally, if there is pending stuff to review, the icon has a red
background.

We have also removed the list of links below a post with the flag
status. A reviewer is meant to click the number beside the flag icon to
view the flags. As a consequence of losing those links, we've removed
the ability to undo or ignore flags below a post.

db/migrate/20190503180839_remove_like_count_from_post_menu.rb

@@ -0,0 +1,9 @@
+class RemoveLikeCountFromPostMenu < ActiveRecord::Migration[5.2]
+  def up
+    execute(<<~SQL)
+      UPDATE site_settings
+      SET value = REGEXP_REPLACE(REPLACE(REPLACE(value, 'like-count', ''), '||', '|'), '^\\|', '')
+      WHERE name = 'post_menu'
+    SQL
+  end
+end