Exempt personal repositories from repository deletion policy

Currently, admins can forbid repository admins to delete repositories globally. More on [repository delete policies|https://confluence.atlassian.com/bitbucketserver/global-permissions-776640369.html] 

However, some users do forks which has to be deleted after changes merged back into the main repository. Others create personal repositories to test new settings for Jenkins builds. These users are unable to remove junk repositories and have to ask admins for help.

It would be great to add one more setting to the repository delete policy:

• Allow users only to delete their own personal repositories.