Url import: rewrote web page for Mercurial
This is a follow-up to !18 (merged)
An improvement over regular GitLab is that the timeout time is the correct one instead of the hardcoded 15 minutes.
Sadly, Mercurial does not have a font-awesome icon (that's what the icon() GitLab helper relays too), but the Git one wasn't really interesting anyway.
We may later on add the official Mercurial icon back.