Add ability to restrict Rails actions for each user

[natacha-beck]

The main admin user (and only that user, not even other admins) should have the ability to selectively disable rails actions for other users. In particular, this could be used to restrict another admin user to only a subset of the administrative tasks that exist. The interface would basically look like a table, where (grouped by controller) the list of actions would be presented with checkboxes. A bit of Rails introspection would be needed to build the table.
Controller Action Enabled?
users create X
users delete X
users show O

The default behavior is that all actions are enabled. When an action is disabled, then the application controller (maybe in one of its 'before' or 'around' filters) would return a proper HTTP error code.