R1:5d8b75b4da97

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…