Add haml-lint.

Extends rubocop to cover the code lurking in haml files. Doesn't work for
us yet, see https://github.com/causes/haml-lint/issues/13
Add a preference to control profile visibility.

Refs #316, but the hiding still needs to be implemented.
Add pry, for easier debugging.
Upgrade rubocop control file.
Implement profile hiding, with controller tests. Refs #316
Ensure thread creators only get subscribed once.

The observers fire before the next line in the controller, so it was
possible for a thread creator to be subscribed for other reasons and
the thread creator subscribe methods were missing the guards.

Fixes #318 Fixes #310
Update changelog.