Switch plasma-mobile to release tarballs

Release tarballs have the benefit that they contain the translations and are better accessible via Yocto, since one can use a full offline mirror.

The previous option to use release tags instead of tarballs still exists. For plasma-mobile we have the situation that apps are not all organized in a single invent group but scattered over several KDE modules. For solving this, just use the legacy anongit:// URL rewrite support with hopefully will stay for still quite a few years.