When adding the enhanced HTML/URL sanitization for Markdown rendering in

Review Board 3.0.22, we broke a particular use case from one of our

users. They were making use of eclipse:// URLs, which linked to some

part of their Eclipse setup. The only URL protocols allowed were

http://, https://, and mailto:, and anything else got basically

stripped away or turned into text.
It's likely this user's use case isn't unique. To support it, we're

introducing an advanced setting in settings_local.py:

ALLOWED_MARKDOWN_URL_PROTOCOLS. This can be set to a list of URL

protocols considered safe, and will supplement the defaults.
There's no configuration for this, and it's not currently documented

outside of an entry in settings.py, but it'll be available as a

feature in 3.0.24.

Unit tests pass.