add a guard to accessing current_user in is_user?
Split up questions#accept_answer into two methods

Clicking to unaccept a question now calls questions#unaccept_answer,
which simply removes the association. Accepting goes through
questions#accept_answer as before.
Merge branch 'rework-accept-answers'

Conflicts:
	app/helpers/users_helper.rb
	app/views/users/show.html.erb