Add setting to define default repository to open pull requests against

Feature Request is to add a setting to define what the default repository to open pull requests against. This is especially helpful for novel uses of forks.

For example, in my use case we have a fork of a legacy software that receives updates and patches. Since this project does not utilize dependency management, we fork this copy of it's base code and make our modifications for each individual project to that.

99.99% of the time when we want to make a pull request on one of these projects it is against it's own fork and not the master fork. However the default in bitbucket is to open the pull request against the master fork.

That's quite a logical default but is very annoying when it cannot be modified.

Therefore my request is to add a setting to set the default project for opening pull requests against.

Tangentially related to my previous request, BCLOUD-11283 (BB-13552)