user/mumble: new package

This build does not include ICE or mumble-overlay support.

make options were changed for machines with 1GB or less RAM due to the demands of compiling in parallel. At -j1, a Raspberry Pi 3 B takes ~90 minutes to build the package, and 3 hours or so to build and test it. Without -j1, it doesn't seem to get past one point or another (generally in translations), as jobs start to compete for resources. This is recognized by upstream: https://github.com/mumble-voip/mumble/issues/3397

Feedback in IRC was adapted and applied as suggested. Some comments were left in, in case someone later wants to re-add overlay or ICE support.