Fix JSON API

[ernestask]

It’s just completely nonsensical, because we:

• try to return JSON-unserializable types:
  

  faf/src/webfaf/reports.py

  Line 590 in 5e15cfc

  solutions=solutions,

  
  (though for that you can use/extend WebfafJSONEncoder)
• try to return JSON-unserializable types that don’t make any sense in the context:
  

  faf/src/webfaf/problems.py

  Line 495 in 5e15cfc

  components_form = ProblemComponents()

• return wildly inconsistent data (null, [null] for the same field, where you would expect [] in case there is no data for an array type)

In short: we should have a well-defined API and not mix endpoints that return JSON or Flask-rendered HTML based on the temperature outside, because we don’t have the tests to verify the correctness (over time as well) and because there should be a clear separation between the backend and web UI, due to its optional nature.

One attempt was #886, but that broke everything and I’ve since discovered that it goes deeper.