We had two places where file attachment comments were being serialized:
In ReviewUI, and in a template tag. These were inconsistent and led to
problems in text/rich text information, depending on the call site.

The template tag now calls out to the review UI for serialization,
ensuring we only do this in one place. That serialization has also been
updated to store rich text information and to do normalization of the
text for edit.

Unit tests pass.
Tested this manually along with another change, and ensured that file

attachment comments always had the correct initial state.