'Automatically configure delete branch checkbox' applies even when branch deletion isn't permitted

To reproduce:

• Set up branch permissions on a repository such that a certain branch cannot be deleted.
• Set up an 'Automatically configure delete branch checkbox' handler on that same repository.
• Create a pull request from the undeletable branch to some other branch.
• Open up the Merge dialog on the PR.

Expected: The checkbox for deleting the source branch (which is greyed out) is not checked, and the merge can be performed without errors.

Actual: The checkbox is checked and the merge can be performed, but there is an error message saying that the source branch couldn't be deleted.