When creating an inline, save the current document engine

Summary:
Ref T13513. As part of inline metadata, save the document engine the change is being rendered with.

This will allow other parts of the UI to detect that an inline was created on a Jupyter notebook but is being rendered on raw source, or whatever else.

The immediate goal is to fix nonsensical inline snippet rendering in email on Jupyter notebooks.

Test Plan:

• Created inlines and replies on normal soure code, saw no document engine annotated in the database.
• Created inlines and replies on a Jupyter notebook rendered in Jupyter mode, saw "jupyter" annotations in the database.
• Swapped document engines between Jupyter and Source, etc.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21240