Merge branch 'dropdown-filter-after-load' into 'master'

Trigger filtering after ajax is complete in dropdown

![filter](/uploads/1c361d968769eb3fe5cdd05dee497aa6/filter.gif)

Closes #15186

See merge request !3813

app/assets/javascripts/gl_dropdown.js.coffee

@@ -154,6 +154,9 @@ class GitLabDropdown
             @fullData = data
 
             @parseData @fullData
+
+            if @options.filterable
+              @filterInput.trigger 'keyup'
         }
 
     # Init filterable

app/views/shared/issuable/_sidebar.html.haml

@@ -118,6 +118,7 @@
                               Manage labels
                             - else
                               View labels
+                  = dropdown_loading
 
       = render "shared/issuable/participants", participants: issuable.participants(current_user)
       - if current_user