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 not displayed accurately).

Instead, use the modern stuff.

Test Plan:

• Created preview comments and inlines in Differential.
• Edited a Differential inline.
• Submitted main and inline Differential comments.
• Viewed and edited Differential summary and test plan.
• Created preview comments and inlines in Diffusion.
• Submitted comments and inlines in Diffusion.
• Verified Differential now loads and saves to the generalized markup cache (Diffusion is close, but main comments still hold a single-stage cache).
• Verified old Differential comments work correctly with the lightbox.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1963

Differential Revision: https://secure.phabricator.com/D3804