Add comment previews to Maniphest

Summary:
Moves shared code from Differential and Maniphest comment previews into
PhabricatorShapedRequest, and then implements Maniphest previews.

This doesn't implement comment drafts, I'll follow up with that but it requires
this and is completely separable.

This also always shows the preview as "commented" rather than previewing the
actual transaction. I'll follow up with that but I think it will require a
little factoring and this is useful even without transaction details.

I need to tweak the styling a bit too.

Test Plan:
Typed text in Maniphest and Differential. Toggled Differential action. Made
comments.

Reviewed By: rm
Reviewers: rm, tuomaspelkonen, jungejason, aran
CC: aran, rm
Differential Revision: 258