Make login page more clear by adding ASTRONauth

Move keycloak down to "sign in with" section Button names are all the same ("sign in") instead of a mix of log in Removed sign in button from login pages (to avoid confusion)

I followed the general design of the Gitlab login page with a "sign in with" section:

This is the proposed design:

The current design is shown at https://sdc-dev.astron.nl/stageit/account/login/

TODO: the hover follows the DIAS button, which turns it blue, which does not look nice (purely cosmetic flaw)

Closes SDC-748