Web: a merge request cannot be opened from a fork #168

mr create takes a source in another repository; the web form offers only branches of the repo being viewed. A contributor without write access has no browser path to propose a change at all, which is the case the fork flow exists for.

The largest of the web parity gaps: needs a source-repository picker and the access checks that go with it.

Parity: Merge requests "create from a fork".