Add select2-toggleselect to the JavaScript files to load

"select2-toggleselect.js" provides an extension to Select2 that makes
possible to unselect items in a multi-select dropdown by clicking on
them. It seems that its load slipped through when moving things around
in commit 6a470e59; this commit adds it
to the JavaScript files to be loaded in the same position that it should
have had in that commit (based on how the other declarations were
moved).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>