luci-base: template: add translation iterator function

luci-base: template: add translation iterator function

Introduce a new luci.template.parser.get_translations() function which will
iterate all loaded translation entries and pass the to the given callback
function.

This is useful to expose the loaded translations in other formats, e.g. for
wrapping them into JSON feeds.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>