When trying to find a `README.md` in a `.profile` repo, do so case
insensitively. This change does not make it possible to render readmes
in formats other than Markdown, it just removes the hard-coded
"README.md".
Also adds a few tests to make sure the change works.
Fixes#1494.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
