Disallow Deletion of "Production Branch"

Currently Stash allows you to specify a "Default Branch". This cannot be deleted and in gitflow makes sense to be `develop`.

In the branch configuration you can select a "Development Branch" (develop) and a "Production Branch" (master)

It would be nice to have the option to stop the production branch from being deleted.

Example use case:
1) hotfix created from `master`
2) auto-merge pushes to `develop` but fails, so puts in a pull request from master -> develop
3) user fixes conflict and merges in Stash, but forgets to uncheck the "Delete source branch" box (which has a setting to remember it's last value)
4) `master` is auto-deleted from Stash.

This seems to be a fairly easy workflow to be able to accidentally delete master and seems a tad risky to me! As long as someone has an up-to-date version of master on their machine they can push it up again, but what if they don't???