Web: a repository cannot be forked #174

repo fork is CLI-only and has no row in the wiki's Parity matrix at all, so it does not read as outstanding work anywhere.

It is the missing half of #168. That gave the merge request form a source picker offering branches of any fork the viewer can push to, but the fork itself still has to be made from a terminal, so the browser path for a contributor without write access stops at the first step: fork, push a branch, propose.

Web edits land in a branch of a repository you can write, so with a fork made in the browser the whole flow — fork, edit a file, open the merge request — is available to someone who has never installed the CLI.

Needs a control on the repository header, the visibility and quota rules the command already applies, and a Repositories row in the matrix.

Ref #168.