Introduced caching of available columns in query service.

Solves problem of select2 not having available values when loading the columns modal.

app/assets/javascripts/angular/controllers/dialogs/columns.js

@@ -63,9 +63,9 @@ angular.module('openproject.workPackages.controllers')
 
   // Available Columns
   QueryService.getAvailableColumns()
-    .then(function(data){
-      $scope.availableColumns = data.available_columns
-      $scope.availableColumnsData = data.available_columns.map(function(column){
+    .then(function(available_columns){
+      $scope.availableColumns = available_columns
+      $scope.availableColumnsData = available_columns.map(function(column){
         return { id: column.name, label: column.title, other: column.title };
       });
     });

app/assets/javascripts/angular/services/query-service.js

@@ -89,15 +89,23 @@ angular.module('openproject.services')
 
     getAvailableUnusedColumns: function(projectIdentifier) {
       return QueryService.getAvailableColumns(projectIdentifier)
-        .then(function(data){
-          return QueriesHelper.getAvailableColumns(data.available_columns, QueryService.getSelectedColumns());
+        .then(function(available_columns){
+          return QueriesHelper.getAvailableColumns(available_columns, QueryService.getSelectedColumns());
         });
     },
 
     getAvailableColumns: function(projectIdentifier) {
+      // TODO: Once we have a single page app we need to differentiate between different project columns
+      if(availableColumns.length) {
+        return $q.when(availableColumns);
+      }
+
       var url = projectIdentifier ? PathHelper.apiProjectAvailableColumnsPath(projectIdentifier) : PathHelper.apiAvailableColumnsPath();
 
-      return QueryService.doQuery(url)
+      return QueryService.doGet(url, function(response){
+        availableColumns = response.data.available_columns;
+        return availableColumns;
+      })
     },
 
     getSelectedColumns: function() {
@@ -228,6 +236,10 @@ angular.module('openproject.services')
       });
     },
 
+    doGet: function(url, success, failure) {
+      return QueryService.doQuery(url, null, 'GET', success, failure);
+    },
+
     doQuery: function(url, params, method, success, failure) {
       method = method || 'GET';
       success = success || function(response){