Strong parameters

Add the strong parameters pattern to all of our controllers.

Points of interest:

- The six calls to `attr_accessible` have been removed.
- `ActionController::Parameters#reverse_merge` has been [monkey patched](https://github.com/alphagov/whitehall/commit/2b3e69a7df910acebeb82e955b670927a07e3bb6) to keep the same internal state of `permitted`.
- `User#attr_accessible` has been [monkey patched](https://github.com/alphagov/whitehall/commit/a13a719118048c3fb296706a857846847b47ad07) to stop the method call in the GDS SSO gem messing things up.