[mod] add i18n infrastructure for SearXNG message files (searxng.msg)

With this patch ``searxng.msg`` files can be added to SearXNG.  In
``searxng.msg`` files messages can be defined which are not captured by babel's
gettext, like the generic names of the categories or messages that are stored in
constants.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>