R1:27cfd8d19e83

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.