ui: Add basic i18n support

We don't support all the gettext niceties like proper plurals etc., but
that should be good enough for now. I have to figure out if Transifex
and its JSON output format supports this, and how.
Move common app parts in a separate package

That avoid dependency loops with the services like thumbnailer or
monitor.
Move path normalization to app/common
Two go fmt fixes

I really need to install that hook that prevents comitting badly
formatted code...
Remove stale 'public' route