Create & edit delegatable permissions

- made the error handling around supported permissions form more railsy
- some refactoring in the supported permissions controller to make it DRY
Freeze signin permission name

Show the signin permission right at the top when
showing permissions supported by an application
Removing bold labels, keeping it consistent
More readable validation
Removing signin permission name text-box
Following convention
Merge pull request #121 from alphagov/141-create-edit-delegatable-permissions

[#141] Create & edit delegatable permissions