Prepare for entity-type-agnostic diffs

Problem/Motivation

Over in #2452523: Offer a revisions tab for all entities, work is almost complete for expanding the diff module to handle all eligible entity types. There is some work in this module that should happen to prepare for that.

1. Update the diff controller check to apply to more than just nodes
2. Fix previous / next URL generation for non-nodes
3. Prevent WSOD if an entity type upcasts the left_revision or right_revision route parameters
4. Add a null check for the author
5. Add test coverage for a non-node (taxonomy, block content, and media should work)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes