Refactor config models to dataclasses

[stsewd]

Currently, we are using namedtuples. A problem with this is that you can't have default values when we move to python3 in production (3.7) we can refactor those to dataclasses.

Pros:

• Default values
• Type hints
• Less repetition
• Our code looks more cool 😎

ref #4456 (comment)

[ba11b0y]

Taking this up. Confirming that the dataclasses should be immutable?
Also where do I refer for the defaults?

[stsewd]

@invinciblycool we don't run python 3.7 in production yet, I'll mark this issue as blocked till that.

Answering your questions, yes, it should be immutable, we can search the default here https://github.com/rtfd/readthedocs.org/blob/master/readthedocs/config/config.py

[stsewd]

We move to use plain classes + __slots__ for now, closing.