Allow any EditEngine comment form to be pinned

Summary:
Fixes T12049. This expands "Haunted" comment panels to EditEngine, and by extension to all EditEngine applications.

Eventual goal is to remove custom commenting code in Differential and replace it with EditEngine code.

Changes from current "haunt" mode:

• This only has one mode ("pinned"), not two ("pinned", "pinned with preview"). There's an inline preview and scroll behavior is a little better.
• Now has a UI action button.

Slightly tricky stuff:

• This interacts with "Fullscreen" mode since it doesn't make sense to pin a full-screen comment area.
• This should only be available for comments, not for remarkup fields like "Description" in "Edit Task".

Test Plan:

• Pinned/unpinned in Maniphest.
• Pinned/fullscreened/unfullscreened/unpinned.
• Checked that "Edit Task" doesn't allow pinning for "Description", etc.
• Pressed "?", read about pressing "Z".
• Pressed "Z".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12049

Differential Revision: https://secure.phabricator.com/D17105