Login screen customization

I want to made a custom login screen to my seafile server (I want to add a new button with custom acction).
I want to add users with phone number, and I need a button, that send the password in SMS to users.
With this solution the user don’t need to remember the password.

I created a pull request:

With this include after extra_script block, with some javascript, we can customize the login page.

Example for custom.html:

<script type="text/javascript">
var button = <button id="custom_button" type="button" onclick="customButtonAction()" class="submit btn btn-primary btn-block"> Send SMS </button>;

$( document ).ready(function() {
  $( "#login-form" ).append(button);
  console.log( "appendOK" );
});

function customButtonAction() {
  alert('custom');
  console.log('custom');
}

</script>