R1:3cbc239bc640

Modernize most somewhat-weird Diffusion controllers

Summary: Ref T4245. This gets everything else except serving HTTP requests (complicated) and lint (quite weird).

Test Plan:
- Viewed a diff.
- Viewed externals.
- Viewed history table to see last modified.
- Did path completion and validation in Owners.
- Did tree path search in Diffusion.
- Viewed a repository.
- Created a new repository.
- Looked up symbols.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential…