Team deploy keys (BB-7170)

What would be super awesome is to have deploy keys that we can set under the team's settings that then apply to all repos under the team.

A use case is that we have an internal server that runs or hubot. This server does our deploying for us and needs read only (deploy key) access to every repo that we use. It's a bit of leg work to have to add the deploy key to every repo as we add new ones.

I was thinking of using the API to do it, but would really like it if it could be a global option for the team.