Add static errors, supported protocols, and a dynamic function listing to external editor settings page

Summary:
Ref T13515.

• Previously valid editor URIs may become invalid without being changed (if an administrator removes a protocol from the list, for example), but this isn't explained very well. Show an error on the settings page if the current value isn't usable.
• Generate a list of functions from an authority in the parser.
• Generate a list of protocols from configuration.

Test Plan:

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21146