Modularize application configuration panels

Summary:
Ref T7149. This is a few steps away, but:

• Generally, I'd like to reduce the amount of "Config" configuration we have.
• One good way to do this is to move it into UIs in Application configuration. We did this with email recently.
• I think this was a great change and I'd like to keep moving in this direction.
• T7149 touches configuration related to file storage engines. Although I'm not planning to fully move configuration into applications yet, it would be easier to debug and test if I could drop a read-only panel there to show engines.
• So, modularize the config stuff so I can add a new panel without hard-coding it.

Test Plan:

• Added, edited, and deleted application emails.
• Viewed non-email application detail pages.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12051