Exposed filters aggregation (regression?)

Rewrote the patch from #2372173. I couldn't quite take the simplified route that was done in 7.x upstream, because the view rendering did not respect exposed filters just being "not exposed" anymore; the user inputs weren't actually in those filters' values.

The approach I did was to cycle through all the filters, look for provided exposed filter values, and implement them. I don't promise it to be foolproof since there isn't any standard sort of "setValue" method, we're just toying with arrays; but for my purposes it works for references and text (direct assignment of ['value']), and dates (if ['value'] is array, set ['value']['value']). Hopefully there is a more foolproof way out there to assign values.

Created a MR to this can be tested. Also moved to the current branch, which is 2.1.x.

I think this needs a test case.

Also, this seems very similar to 🐛 Does not work with data export + paging Active ? How are they different, and will the patch here fix that issue too?