Allow admin to create public deploy keys that are accessible to any project.

Addresses private issue https://dev.gitlab.org/gitlab/gitlabhq/issues/1774.

Project settings:

The "Public deploy keys" section is only shown when there are any. If there are public deploy keys but no project deploy keys, only public deploy keys are shown. If there are no public deploy keys and no project deploy keys, the current "Deploy keys from projects you have access to will be displayed here" placeholder is shown.

The list of projects below the public key has been changed to only show projects the user has access to.

"Public deploy key" seems to be repeated on the left, but the first is just the title. The label is always visible for public deploy keys.

Admin index:

Admin detail page:

Projects using the deploy key are listed on the left and can be disabled easily.