v0.8.1

This is the next pre-release of our 0.8 branch, which includes larger under-the-hood improvements. Changes since the last release:

• Use native threading module in job_manager. (#1096)
• Fix next_job timer logic. By @jayeclark (#1100)
• Remove jobs if scheduler setting is changed. (#1105)
• Remove jobs before deleting profile (#1107)
• Make sure correct sources are removed (#1110)
• Address DB Locking issues (#1109)
• Refactor: Split up vorta/models.py (#1112)
• Notify on failed repo checks (#1113)