support selecting auth method in login dialog

The implementation of #46 (closed) brought a list of possible authorization methods into the login dialog. This list should be evaluated to render a select box (in the given order) and preselect the first element.

During login the currently selected element should be taken into account to perform the login with the desired auth method. If nothing is selected (for whatever reason) the first method is selected.

For calling the login handler (URL "/sign-up/handle/") the password is sent btoa encoded for all non-Basic auth methods.

AC:

• restricted auth methods appear in combo box in given order