feat(issue_platform): Auto import grouptype.py in all Django apps if present

[wedamija]

This pr ensures that if a grouptype.py is implemented in any Django app that we use that it will be imported. This allows us to define GroupType entries outside of issues/ without worrying that they might not be imported.

This allows us to move logic that is specific to given issue types outside of issues/. In retrospect, it was probably not the right move to store the actual definitions here since the platform is meant to be generic and not be aware of the details of any specific issue type.

As well as this, as part of alerts are issues we're planning on hooking in information about detectors into the issue types, which would mean that we'd need to import from uptime/incidents/etc. This would likely result in import loops.