Set "delete source branch" as default for all pull-requests

Hi Atlassian Team,

as far as I can see there is no way to enable the flag "delete source branch" per default for all pull-requests. 

Background: some developers forget to click this checkbox when creating a pull request. After merging the source branch still exists.

It would be great if we could define "delete source branch" as default for all pull requests. So a developer has to unset it if he doesn't want to delete the source branch (which is only for some seldom cases).

Thanks

Matthias