Refactor UsersSelect to use internal gitlab autocomplete controller

app/assets/javascripts/users_select.js.coffee

 class @UsersSelect
   constructor: ->
+    @usersPath = "/autocomplete/users.json"
+    @userPath = "/autocomplete/users/:id.json"
+
     $('.ajax-users-select').each (i, select) =>
+      @projectId = $(select).data('project-id')
+      @groupId = $(select).data('group-id')
+      showNullUser = $(select).data('null-user')
+      showAnyUser = $(select).data('any-user')
+
       $(select).select2
         placeholder: "Search for a user"
         multiple: $(select).hasClass('multiselect')
         minimumInputLength: 0
-        query: (query) ->
-          Api.users query.term, (users) ->
+        query: (query) =>
+          @users query.term, (users) =>
             data = { results: users }
+
+            if query.term.length == 0
+              anyUser = {
+                name: 'Any',
+                avatar: null,
+                username: 'none',
+                id: null
+              }
+
+              nullUser = {
+                name: 'Unassigned',
+                avatar: null,
+                username: 'none',
+                id: 0
+              }
+
+              if showNullUser
+                data.results.unshift(nullUser)
+              if showAnyUser
+                data.results.unshift(anyUser)
+
             query.callback(data)
 
-        initSelection: (element, callback) ->
+        initSelection: (element, callback) =>
           id = $(element).val()
-          if id isnt ""
-            Api.user(id, callback)
-
+          if id != "" && id != "0"
+            @user(id, callback)
 
         formatResult: (args...) =>
           @formatResult(args...)
@@ -38,3 +66,34 @@ class @UsersSelect
 
   formatSelection: (user) ->
     user.name
+
+  user: (user_id, callback) =>
+    url = @buildUrl(@userPath)
+    url = url.replace(':id', user_id)
+
+    $.ajax(
+      url: url
+      dataType: "json"
+    ).done (user) ->
+      callback(user)
+
+  # Return users list. Filtered by query
+  # Only active users retrieved
+  users: (query, callback) =>
+    url = @buildUrl(@usersPath)
+
+    $.ajax(
+      url: url
+      data:
+        search: query
+        per_page: 20
+        active: true
+        project_id: @projectId
+        group_id: @groupId
+      dataType: "json"
+    ).done (users) ->
+      callback(users)
+
+  buildUrl: (url) ->
+    url = gon.relative_url_root + url if gon.relative_url_root?
+    return url