Yet another attempt to remove APIv3 magic

[FrostyX]

An attempt number 356 to remove magic and excess complexity from our APIv3 schemas. I won't sleep soundly until we figure it out, so in order to preserve my own sanity, here we go.

• I am keeping schemas as @dataclass, we already figured out that it is what we want
• I am keeping shared fields in fields.py but moving only once used fields definitions directly to schemas
• I am removing some magic:
  • It is no longer possible to mark all fields as required. The largest schema that did it had 3 fields.
  • We are no longer magically matching schema fields by their name with their definitions in fields.py
  • Because of this ^^, we don't need the hack for renaming field names
• I simplified a lot of code in Schema, InputSchema, and ParamsSchema

When reviewing, I recommend going commit by commit.

In a separate follow-up PR, I'd like to take a look at the schema class hierarchy.

[nikromen]

+1, very nice improvement and keeping the dataclasses will help us with fedora-copr/debate#9 (if we will ever get there :D)