Provide the ability to cancel an import of a repository

New feature summary

Provide the ability to cancel an import of a repository
There is currently no way to cancel an on-going repository import.
It would be nice to have the ability to cancel an import, preferably via the UI.

This can be useful for situations where an import is taking too long or has become stuck due to an issue.

Current workaround

To cancel an import, the “Bitbucket Server - Importer” System plugin can be temporarily disabled.
This action cancels all on-going imports

Procedures:

• In the Bitbucket UI, go to Administration >> Manage apps
• Select “System” from the dropdown to list system apps
• Look for “Bitbucket Server - Importer” and disable it.
  Disabling this plugin cancels all imports
• Afterwards, re-enable the plugin