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
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D8551
