Shuffle various parts of the config edit UI around

Summary:
Fixes T9339.

• Don't show edit control for locked config at all.
• Don't show a "Cancel" button either.
• Change "Value" label to "Database Value" for non-custom config.
• Highlight effective value.
• Move examples under current state.
• Tweak some formatting.

Test Plan: {F777878}

Reviewers: chad, avivey

Reviewed By: chad, avivey

Subscribers: avivey

Maniphest Tasks: T9339

Differential Revision: https://secure.phabricator.com/D14054