Allow EditEngine configurations to be disabled and marked as "Default"

Summary:
Ref T9132.

Let configurations be enabled/disabled. This doesn't do much right now.

Let configurations be marked as default entries in the application "Create" menu. This makes them show up in the application in a dropdown, so you can replace the default form and/or provide several forms.

In Maniphest, we'll do this to provide a menu something like this:

• New Bug Report
• New Feature Request
• ADVANCED TASK CREATION!!11~ (only available for Community members)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14584