Add translation caching & support for translating Discussion, Motion, and Vote

Conflicts (resolved):
	Gemfile
	Gemfile.lock
	app/controllers/comments_controller.rb
	app/models/translation.rb
	app/models/user.rb
	app/models/vote.rb
	app/views/comments/_comment_likes.html.haml
	app/views/discussions/show.html.haml
	app/views/motions/_motion.html.haml
	app/views/motions/_votes_table_vote.html.haml
	db/schema.rb
	spec/models/ability_spec.rb