R1:c79094d7a8bd

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…