Follow-up to #2587755: AJAX error when using progress bar on file field widget
Problem/Motivation
The file module's FileWidgetAjaxController class is a bit ugly/messy.
Solution
Refactor. Mostly drafted by chx, discussed on IRC (so he should get a credit on commit assuming this gets commited). There was also some discussion on eligibility for commiting into 8; no conclusion was reached but brought up was:
- until/before #2587755: AJAX error when using progress bar on file field widget it didn't work (returned a 500 HTTP response)
- therefore there shouldn't be any disruption
- controllers are not API surface
- Any contrib usage would have had to change the path so probably would have just created a new route and as long as it passed the key to this class it would work with this refactoring.
