Add drafts to Maniphest

Summary:
Use PhabricatorDraft to save text when previewing it so you don't lose stuff if
your browser crashes.

Test Plan:
Typed some text, closed/reopened the page, text was still there. Submitted
comment, text vanished.

Reviewed By: tuomaspelkonen
Reviewers: rm, tuomaspelkonen, jungejason, aran
CC: aran, tuomaspelkonen
Differential Revision: 262