Makefile: add target 'useragents.update' to fetch versions of Firefox

Update searx/data/useragents.json with the most recent versions of Firefox.

BTW: add 'useragents.update' to 'project' target and clean up the Makefile and
remove it from the manage.sh script.

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

closes: https://github.com/asciimoo/searx/issues/1882