Add no-op default action to login form

That way people clicking too fast on login before the view models are
bound won't be able to make the form submit username and password as GET
parameter to the base URL.

Closes #2108

src/octoprint/templates/navbar/login.jinja2

@@ -3,7 +3,7 @@
     <b class="caret"></b>
 </a>
 <div id="login_dropdown_loggedout" style="padding: 15px" class="dropdown-menu" data-bind="css: {hide: loginState.loggedIn(), 'dropdown-menu': !loginState.loggedIn()}">
-    <form id="loginForm" data-bind="event: {'submit': loginState.prepareLogin }">
+    <form id="loginForm" data-bind="event: {'submit': loginState.prepareLogin }" onsubmit="return false; // this gets overwritten on view model bind">
         <label for="login_user">{{ _('Username') }}</label>
         <input type="text" id="login_user" name="username" data-bind="valueWithInit: loginState.loginUser" placeholder="{{ _('Username') }}" autocapitalize="none" autocorrect="off">
         <label for="login_pass">{{ _('Password') }}</label>