Fix a regression disabling syntax highlighting in the API.

Review Request #12887 — Created March 14, 2023 and submitted — Latest diff uploaded


Review Board


We just recently moved to a new internal configuration model for
controlling the rendering pipeline, but one call location (the FileDiff
API) was still using the old method. That'd be fine, except the old
method wasn't properly converting a request to disable syntax

This led to requests for diff data to return HTML versions of lines
instead of plain text.

The settings migration code now allows for disabling settings, and the
API has been updated for the new settings code.

Unit tests check to ensure that we're properly getting plain-text back
from it, unless requested otherwise.

Verified that the API returned plain text data for any diffs I queried.

A customer verified this fixed a regression they noticed.

Unit tests passed.