Add custom protocol whitelisting to SanitizationFilter

Addresses internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2613

We allow any protocol for autolinks: irc://irc.freenode.net/git

But manual Markdown links with the same protocol get sanitized: [This will not be clickable](irc://irc.freenode.net/git): This will not be clickable

To get around this we have to first allow all protocols, and then manually clean dangerous (i.e., javascript:) protocols.