🚚(backend) split api modules to improve readability

[qbey]

Purpose

The core.api.viewsets module and the core.api.serializers module are getting bigger: I propose to split them into submodules.

Proposal

Use the https://pypi.org/project/gitfilesplit/ tool to split files without losing the history.

• [x] Split core.api.viewsets
• [x] Split core.api.serializers

[Morendil]

Well that's my "Today I Learned" (TIL) - hadn't heard of GitFileSplit or this technique of using octopus merges. Thanks ;)

[qbey]

Now is not the right time to do it, there is too much work conflicting in another PR...