Remove username/password forms from login pages

See our discussion at https://open.greenhost.net/openappstack/openappstack/merge_requests/210/diffs#note_14817

I wrote:

for users it's confusing to have different login mechanisms and I hoped we could eliminate the username/pw login completely. I know i.e. different sites like github/gitlab support both, and that's neccessary because it's not a closed system. But in a controlled environment like OAS we shouldn't have username/password login forms, at least not at the main login page (maybe under a different path like /admin).

Lets continue our discussion here.