Support object mentions in Calendar Event descriptions

Summary:
Ref T7924. This:

• Adds support for remarkup block changes to Modular Transactions.
• Exposes remarkup changes from the Calendar event "Description" transaction.

This makes stuff like mentions and file embeds work properly.

Test Plan:
Mentioned a task in an event description, saw a mention appear on the task.

Uploaded a file to an event description, saw the file become "Attached" to the event.

(Neither of these worked properly before.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7924

Differential Revision: https://secure.phabricator.com/D16481