muc-views: pass all model attributes to template

Makes it possible to show a validation message in a downstream
application.

src/converse-muc-views.js

@@ -1474,12 +1474,12 @@ converse.plugins.add('converse-muc-views', {
                     __('Choose a nickname to enter') :
                     __('Please choose your nickname');
 
-                const html = tpl_chatroom_nickname_form({
+                const html = tpl_chatroom_nickname_form(Object.assign({
                     heading,
                     'label_nickname': __('Nickname'),
                     'label_join': __('Enter groupchat'),
-                    'nickname': this.model.get('nickname')
-                });
+                }, this.model.toJSON()));
+
                 if (_converse.muc_show_logs_before_join) {
                     const container = this.el.querySelector('.muc-bottom-panel');
                     container.innerHTML = html;

src/templates/chatroom_nickname_form.html

@@ -2,7 +2,6 @@
     <form class="converse-form chatroom-form converse-centered-form">
         <fieldset class="form-group">
             <label>{{{o.heading}}}</label>
-            <p class="validation-message">{{{o.validation_message}}}</p>
             <input type="text" required="required" name="nick" value="{{{o.nickname}}}"
                    class="form-control {{o.error_class}}" placeholder="{{{o.label_nickname}}}"/>
         </fieldset>