Create main branch merge permission

[tyler-8]

Plugin Version

0.5.2

Proposed functionality

Using the existing permissions system, add a new permission for allowing/disallowing "merge into main branch" that can be assigned to users/groups just like other permissions within NetBox.

Use case

This would offer a way for NetBox admins to have a middle-ground between a "wild west of branching" and the commercial offering from NetBoxLabs that adds approval workflows for branching.

Main merging permissions could be granted to just admins, or a specific user group.

I know #189 exists - and I think that's a great path forward as well. However, the plugin route - or even using the validators from #124 - is adding more overhead for something that should fit very well into the existing permissions system usage.

External dependencies

N/A