Incorporate CSS from select2-bootstrap-css & its license information.

RhodeCode 2.2.5 included a modified version of this upstream work. We
include herein the upstream version as downloaded and incorporated using the
following commands:

(cd /tmp; \
git clone https://github.com/t0m/select2-bootstrap-css
)
cp /tmp/select2-bootstrap-css/select2-bootstrap.css rhodecode/public/js/select2/

The content used to be appended to select2.css but we now keep it in a separate
file select2-bootstrap.css . This file could be included in root.html like
select2.css but we do currently not want to use bootstrap.

The license info was incorporated from /tmp/select2-bootstrap-css/LICENSE
into our LICENSE.md file.