Restore OpenSearch support and use TPAC search

This enables users to easily add the Evergreen search engine to their browser's
built-in set of search engines, and most importantly directs searches to the
TPAC instead of the weird old Supercat interface.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>
OpenSearch release note

Add a short release note about the return of OpenSearch support.

Signed-off-by: Ben Shum <bshum@biblio.org>