Use `submit` event in login form

[Ma27]

It's a pet peeve from me when logging into my personal Hydra that I always have to press the button rather than hitting Return after entering my password.

Reason for that is that the form doesn't have a "submit" button, so far it was always listened to the "click" event. Submit does that and you can hit Return alternatively.

cc @dasj @Ericson2314

[mweinelt]

Deployed on hydra.nixos.org. Finally, my password manager is working. Thanks!