Refactor `mllaunchpad.api`

[schuderer]

Description

The module mllaunchpad.api could be easier to maintain than it is now (v1.0.0).

One reason is that duplicates code from mllaunchpad.model_actions, in a way that makes it hard to test and expand. Also, the separation of concerns between objects and functions appears wonky in some places. Maybe the way we use Flask-Restful's resources etc. is one reason for this/could be improved (right now, mixing types of resources lead to some intermingling and more if-else constructs than I personally find appealing).

All of this also the reason that unit tests on api are currently somewhat lackluster and branch coverage is lower compared to other parts of the code base.