Set deployment key as account-wide default (BBCDEV-3005)

Deployment keys are a great feature, but there ought to be a way to set a deployment key as an account-wide default, particularly for team accounts, rather than only being able to set them one repository at a time.

As it is now - having to add the key to every repository - they aren't a great substitute to using a read-only deployment user. Using deployment keys necessitates entering them in for each repo, one at a time, and then remembering to do it again when new repos are created.

The fact that you can set up a read-only deployment user makes this issue less than critical, but it would be nice to not eat up that user slot.