Edit and create timesheet records in modal (#603)

- added loading indicator for datatables
- fix multiple reloads of datatable content

tests/Controller/TimesheetControllerTest.php

@@ -38,7 +38,7 @@ class TimesheetControllerTest extends ControllerBaseTest
         $this->assertEquals(5, count($result));
 
         foreach ($result as $item) {
-            $this->assertEquals('btn btn-default', $item->getAttribute('class'));
+            $this->assertContains('btn btn-default', $item->getAttribute('class'));
             $this->assertEquals('i', $item->firstChild->tagName);
         }
     }