Add canned script to enable delete source branch checkbox to be checked by default on pull request merges

As a Bitbucket system admin/repo admin

I want to be able to configure the delete branch on merge checkbox to be checked by default.

So that developers using Bitbucket are automatically provided with the default option for deletion of the branch.

Acceptance Criteria

This should be added as an "Event" in SR4BB in the same way "Auto add reviewers to pull request" is.

This event should be called "Auto configure delete branch on merge checkbox".

This should be added at both the system admin level, where checkboxes will configure which repos this is applied to and at the repo level. Again, just as "Auto add reviewers to pull request" is.

NOTES

This is a highly voted on issue for Atlassian: https://jira.atlassian.com/browse/BSERV-9254