Make login method post.

Avoids username and pw inadvertently being added to the URL if the submit event
handler for some reason fails (which shouldn't happen of course).

src/templates/login_panel.html

-<form id="converse-login">
+<form id="converse-login" method="post">
     <label>{{label_username}}</label>
     <input type="username" name="jid" placeholder="Username">
     <label>{{label_password}}</label>