Added ability to control whether users can delete posts.

Similar to the currently existing ability to control whether users
can edit posts.

Fixed post view, so buttons reflect whether ability exists or not.

Added tests.

Fixes #485

Conflicts:
	spec/controllers/posts_controller_spec.rb
Added topics that need moderation to view.

These were found in the controller, but were not
displayed in this view.  Now they are listed as well.

Created tests.

Fixes #484