Cleaned up PostsController#edit specs.

All specs for all controllers now enabled. All controller actions now
use new `Actions` command objects.

Also
* corrected typo in error message in
EditPost#broadcast_guest_not_authorised;
* PostsController#on_edit_failure was added;
* 'an unauthorised user for this post' shared example updated to use
optionally specified message; this lets us use different messages for
guest user and non-author registered user.

RSpec: 1109 examples, 0 failures, 47 pending
RuboCop: 255 files inspected, no offenses detected