[WIP] Implementing lazy load feature for review request

Review Request #11433 — Created Feb. 5, 2021 and updated — Latest diff uploaded

Review Board
reviewboard, students

ReviewBoard currently renders all historic data for review request,
and the data ends up getting computed and loaded on the client, which
makes things seem slow. In order to enhance performance, we would like
to implement a lazy-load feature where collapsed review entries get sent
to the client empty, and then fetch the content dynamically when and if
the entry is expanded.

When we inspect elements on the browser, any entry boxes that is collapsed
will not render the body tag (the body tag will be empty).