Revert "Upgrade select2.js to 79b5bf6db"

This reverts commit 4400040360b78cf4bf4f27561169468b27fabc55.

The select2 upgrade erroneously removed the 'add all current
selections' functionality that was added specifically. Upgrade adds no
features, removes features, therefore, revert.
Merge pull request #21 from anchor/select2_revert

Revert select2.js upgrade