Remove prominence column.

Add a migration to remove the unused column 'prominence' from
info_request_events
Re-annotate models with database fields
Remove validation of prominence
Remove unused InfoRequestEvent.user_can_view? method.
Remove references to InfoRequestEvent.prominence in spec fixtures.
Remove reference to InfoRequestEvent.prominence.
Add validation of prominence
Add response_event helper

Add a convenience method for getting the 'response' event associated
with an incoming message.
Refactor correspondence template into partials.
Rename method to make it a bit more generic

A super user will be able to see all hidden things, not just requests.
Add a method to ask whether a user can view an incoming message.
Refactor common logic about prominence and access.

Move it into the Ability module.
Refactor some common setup steps in integration tests into a DSL.

Add a failing test for what should happen on request hiding.
Add messages for hidden and requester_only states.

Different messages for normal user, requester and admin user.
Update model annotation.
Adding prominence_reason to IncomingMessage.
Add prominence reason.
Remove unused hidden correspondence template.
Don't index hidden and requester_only incoming messages.
Move outgoing message admin actions to their own controller.

Eventually this should use standard RESTful routing.