R1:4b43667086ae

Introduce PHUIRemarkupView, a sane way to work with Remarkup

Summary:
Fixes T9273. Remarkup has reasonably good fundamentals but the API is a giant pain to work with.

Provide a `PHUIRemarkupView` to make it easier. This object is way simpler to use by default.

It's not currently as powerful, but we can expand the power level later by adding more setters.

Eventually I'd expect to replace `PhabricatorRemarkupInterface` and `PhabricatorMarkupOneOff` with this, but no rush on those.

I converted a few…