Add 'post as moderator' option for comments.

* Add 'post as moderator' option to comment form. This creates a so-called sticky comment.
* Downvotes have no effect on stickied comments; they're always visible, regardless of comment thresholds.
* Only mods may sticky comments.
* Mods may sticky comments by other users.

db/migrate/20161227003428_add_sticky_to_comments.rb

@@ -0,0 +1,5 @@
+class AddStickyToComments < ActiveRecord::Migration
+  def change
+    add_column :comments, :is_sticky, :boolean, null: false, default: false
+  end
+end