Slightly modernize all Diffusion edit endpoints

Summary: Ref T4245. Prepares edit endpoints for more flexible repository identifiers.

Test Plan:

• Added, edited, deleted mirror.
• Created repository.
• Edited basic repository information.
• Edited policies for a repository.
• Activated/deactivated repository.
• Updated a repository.
• Hit "Delete" dialog for a repository.
• Edited hosting.
• Toggled dangerous changes.
• Edited branches.
• Edited automation.
• Tested automation.
• Edited storage.
• Edited staging.
• Edited encoding.
• Edited symbols.
• Edited branches.
• Edited actions.
• Tried to do edits as an unprivileged user.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14945