Reenable import from external hosting feature
In the early prototype days of heptapod, the import options at creation of a new repo has been disabled, simply because supporting it was an unneeded effort and didn't help prove the functional feasability of GitLab + Mercurial.
Now it's time to have it back, it's a must to initialize big repositories for which a direct push would exceed any sensible timeouts. See also #25 (closed)