Add a mechanism to automatically delete merged branches after some time

[jeremystretch]

Plugin Version

v0.2.0

Proposed functionality

Implement a configurable setting which automatically deletes merged branches after some pre-defined amount of time has passed (similar to how NetBox can clean up old changelog entries). This should be disabled by default, as deleted branches can no longer be automatically reverted.

Use case

This will automatically clean up branches which are no longer needed and help reduce database sprawl.

External dependencies

No response