TPAC: Implement a locale picker

TPAC: Implement a locale picker

In situations in which more than a single locale is configured, display
a locale picker in the TPAC header based on the registered locales. We
set the eg_locale cookie if passed a set_eg_locale GET param. Default
the selection to the currently selected locale (if any) and resubmit the
current page request.

Grabs the localized locale names, if available, from the database;
otherwise falls back to the en-US version of the locale names.

The locale picker form resubmits the current page with any variables
that were present in the current page request, so that switching locales
should still maintain state.

Signed-off-by: Dan Scott <dscott@laurentian.ca>