Add .css ext for non-SASS file

Author: fbacall

app/assets/stylesheets/application.scss

@@ -47,7 +47,7 @@
 // Vendor
 @import "cytoscape-panzoom";
 @import "jquery.simplecolorpicker";
-@import "bootstrap-datetimepicker";
+@import "bootstrap-datetimepicker.css";
 @import "ardc_vocab_widget_v2";
 @import "jquery.qtip.min";
 @import "select2";