DiffField is now responsible for recording change entries for new diffs,

and for showing those entries. This logic is no longer hard-coded into

the template, review_detail view, or ReviewRequestDraft.publish.
Since this (and other similar upcoming fields) require access to state

generated during the review_detail view, I've added an internal

BuiltinLocalsFieldMixin that takes in a locals() and extracts necessary

data from it. This is only meant for internal usage.
The rendered output from DiffField looks the same as it did in the

template. An upcoming change for better change entry rendering will

improve this output a bit.

Uploaded a few diffs, publishing after each one. Saw that they were recorded

and displayed properly, all with valid links.
Published changes without diff uploads and saw that the diff wasn't recorded or

rendered.
Unit tests pass.