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 Revision: https://secure.phabricator.com/D14947