Before the push git would make a call to
`/:namespace/:project/git-receive-pack`. This would perform an access
check without a ref. So the `Project#branch_allows_maintainer_push?`
would return false.

This adjusts `Project#branch_allows_maintainer_push?` to return true
when passing no branch name if there are merge requests open that
would allow the user to push.

The actual check then happens when a call to
`/api/v4/internal/allowed` is made from a git hook.

Allow builds that have been triggered by a user before terms were
enforced access to git. That way the builds can complete as usual.

Removes permanent redirects, this means that redirects will only be
possible as long as the old route isn't taken by a new project/group.