Disallow deleting branches that are targets of open PRs

When closing a branch through a PR or deleting it in the UI or command line  there are no checks to see if there are other open PRs where this branch is participating as a source or destination.

There should be a way to either:

1. Remove the ability to close/delete the branch that is also part of other open  PRs
2. Automatically delete the PRs associated with deleted branch