Bug Fix: CommentsController#update update_attribute
Integrates commenting for amendments (polymorphic)

Implements a polymorphic association between proposals
amendments and comments
Embed routes for making comments only on proposals
or only on amendments with no overlap
Abstract comment creation logic to fit all commentables

Proposals and Amendments are commendable, use a private
method to extract which so that the controller can
associate the comment correctly
Updates Comment Views

Creates buttons that hit the newly created routes
to create or delete comments