R1:039b8e43b98c

Whitelist allowed editor protocols

Summary:
This is the other half of D8548. Specifically, the attack here was to set your own editor link to `javascript\n:...` and then you could XSS yourself. This isn't a hugely damaging attack, but we can be more certain by adding a whitelist here.

We already whitelist linkable protocols in remarkup (`uri.allowed-protocols`) in general.

Test Plan:
Tried to set and use valid/invalid editor URIs.

{F130883}

{F130884}

Reviewers: btrahan

Reviewed By: btrahan…