FIX: Don't show "bulk" icon on group-index for unauthorized users (#12166)

Closes https://meta.discourse.org/t/group-management-tools-show-up-for-all-users/179259/9

Includes:
* DEV: Add missing <tr>

app/assets/javascripts/discourse/app/templates/group-index.hbs

@@ -23,8 +23,11 @@
     {{#load-more selector=".group-members tr" action=(action "loadMore")}}
       <table class="group-members">
         <thead>
+          <tr>
             <th class="bulk-select">
+              {{#if canManageGroup}}
                 {{flat-button class="bulk-select" icon="list" action=(action "toggleBulkSelect") title="topics.bulk.toggle"}}
+              {{/if}}
             </th>
             {{#if isBulk}}
               <th class="bulk-select-buttons">
@@ -46,6 +49,7 @@
                 }}
               {{/if}}
             </th>
+          </tr>
         </thead>
 
         <tbody>