Remove 'drf-nested-routers' dependency

[stephenfin]

Overview

Patchwork currently has very few non-testing dependencies. Most of the dependencies it does have can usually be found upstream with one exception: drf-nested-routers. We should come up with a strategy to remove this dependency. I see two options:

1. We bring the source for drf-nested-routers in-tree. It's rather simple and hasn't changed much in years, so this should be easy to do
2. We introduce a little verbosity and manually define our routes, instead of using a router. This is what HyperKitty does.

Dependencies

None.

Additional Work Items

None.

Alternatives

We keep things as they are.

Useful Resources

None.