Currently, if you have a mirrored repo, the button on the "branches"
page to create a new branch is available to be pressed. Once you name
your new branch and click submit, you get a 404 page that doesn't explain
what went wrong.
As new branch creation is not supported on mirrored repos, let's just
take that button away if the repo is a mirror. This is already done for
archived repos, so we just need to add another check.
Fixes#7639.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7640
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: John Moon <john.moon@vts-i.com>
Co-committed-by: John Moon <john.moon@vts-i.com>
