[FIX] web_editor: use valid bootstrap for m2o editor options

The .dropdown is in a .btn-group environment while a .btn-group can
only contain .btn or .btn-group elements.

This induced problems where the m2o options covers the full overlay
environment, preventing clicks on the customize dropdown.

addons/web_editor/static/src/xml/snippets.xml

@@ -60,7 +60,7 @@
     <!-- t-field options -->
 
     <t t-name="web_editor.many2one.button">
-        <div class="dropdown">
+        <div class="btn-group">
             <a href="#" class="btn btn-default btn-sm dropdown-toggle" data-toggle="dropdown" data-hover="dropdown" title="Search Contact"><i class="fa fa-search"></i></a>
             <ul class="dropdown-menu contact_menu" role="menu">
                 <li><a><input href="#" type="email" placeholder="Search"/></a></li>