Check for open changes on branch deletion

Check for open changes when deleting a branch in the Gerrit WebUI.
Delete a branch only if there are no open changes for this branch.

The motivation for this change is to make the user aware of open
changes when deleting a branch.

Change-Id: Ifa5068dc5f93a48ced69b171465193ee35d5ab3b
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
7 files changed