muted_user_settings: Add table-sticky-headers class to table header.

table-sticky-headers class is used to make the header of the
table fix at the top.

static/templates/settings/muted_users_settings.hbs

@@ -2,7 +2,7 @@
     <input id="muted_users_search" class="search" type="text" placeholder="{{t 'Filter muted users' }}" aria-label="{{t 'Filter muted users' }}"/>
     <div class="progressive-table-wrapper" data-simplebar data-list-widget="muted-users-list">
         <table class="table table-condensed table-striped wrapped-table">
-            <thead>
+            <thead class="table-sticky-headers">
                 <th data-sort="alphabetic" data-sort-prop="user_name">{{t "User" }}</th>
                 <th data-sort="numeric" data-sort-prop="date_muted">{{t "Date muted" }}</th>
                 <th class="actions">{{t "Actions" }}</th>