frontend: first steps towards removing unnecessary APIv3 abstractions

[FrostyX]

We intentionally left a huge mess and a lot of magic in fields.py, schemas.py, and docs.py because it made the migration to Flask-RESTX a lot easier and we didn't know the correct abstractions beforehand.

In this patch, I am proposing several refactors to simplify the code:

• Schemas duplicated file types that were already-defined in fields.py. I am using the already defined ones to avoid duplication
• There seems to be no need for separately defining schemas and models now. In the end, we care only about models and schemas were IIRC only an implementation detail. I am showing how to remove some schemas and define models directly.
• Having every field defined as a variable in one giant pile in fields.py was a temporary measure. We can define the fields within models and then point to them to avoid duplication. See project_fork_input_model and its ownername.

[FrostyX]

Do you think this approach could be done for the whole APIv3 code, or we will hit a wall somewhere? The information at the swagger page didn't seem to break and beaker tests mostly pass with exception to:

delete-build 2918198 2918213 2918219

which tracebacks. But I don't think I caused it in this PR (not sure).

[praiskup]

We'll have to wait for the Pydantic support in python3-flask-restx.

The thing Jakub propose works on "outputs", but not on "user inputs" int he API.