Use the unified markup cache for Maniphest

Summary:

• See D2945.
• Drop cache field from ManiphestTransaction.
• Render task descriptions and transactions through PhabricatorMarkupEngine.
• Also pull the list of macros more lazily.

Test Plan:

• Verified transactions and transaction preview work correctly and interact with cache correctly.
• Verified tasks descriptions and task preview work correctly.
• Verified we don't hit the imagemacro table when we're rendering everything from cache anymore.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2946