R1:fdf90b46eb88

Use modern two-stage markup cache (PhabricatorMarkupInterface) in Differential

Summary:
See T1963 for discussion of the Facebook-specific hack.

Differential currently uses a one-stage cache (render -> postprocess -> save in cache) rather than the two-stage cache (render -> save in cache -> postprocess) offered by `PhabricatorMarkupInteface`. This breaks Differential comments coming out of cache for the lightbox, and makes various other things suboptimal (status of handles like @mentions and embeds are…