Default services for all repos in account (BB-4489)

It should be possible to define services at the account level (particularly now with team accounts), and have those service settings propagate to all repositories on that account.

For example, my team uses HipChat and we obviously want all of our repos to use the HipChat notification. Right now, that involves a painful one-at-a-time manual process of adding the service to every repo, and remembering to do it again when we add new repos.

Just as we are able to define user groups so that we don't have to do this manual process for each repository's read/write/admin access, we should be able to define default services so that we don't have to do this for each repo's services.