Only load autocomplete data when actually needed

Previously this would result in autocomplete data being loaded for every
page (e.g. the page showing all Git branches of a project).

app/assets/javascripts/gfm_auto_complete.js.coffee

@@ -68,6 +68,7 @@ GitLab.GfmAutoComplete =
             title:  sanitize(m.title)
             search: "#{m.iid} #{m.title}"
 
+    if @dataSource
       $.getJSON(@dataSource).done (data) ->
         # load members
         input.atwho 'load', '@', data.members

app/views/layouts/_init_auto_complete.html.haml

 - project = @target_project || @project
-:javascript
+
+- if @noteable
+  :javascript
     GitLab.GfmAutoComplete.dataSource = "#{autocomplete_sources_namespace_project_path(project.namespace, project, type: @noteable.class, type_id: params[:id])}"
     GitLab.GfmAutoComplete.setup();