Add OARS and tweak SPDX licences

OARS is a standard that allows Software Stores (such as GNOME Software) to apply local ratings to applications. It's a requirement for new apps on Flathub and it'd be great to get it upstream into existing apps as well.

I also tweaked a little bit of the language to read better in english, updated the URLs to gitlab ones to avoid a redirect and made the licences valid SPDX