[Finishes #64521456] Adds friendly_id to Gemfile; updated gem bundle.
[#64521456] Generate friendly_id, adds friendly_id and post migrations; ran db:migrate.
[#64531740] Generate friendly_id, adds friendly_id and post migrations; ran db:migrate.
[#64531740] Generate friendly_id, adds friendly_id and post migrations; ran db:migrate.
[Finishes #64531740] Extends Post model with friendly_id.
[Finishes #64531572] Admin::PostsController and PostsController uses friendly find.
[Fixes #64532676] RedCloth now called only when Post.content is true to avoid TypeError exception.
[Finishes #64531476] Adds rake task to save existing posts.
[Finishes #64531458] Adds uniquness validation for Post.title; updates spec/ to cover Post.title uniqueness.