add (disabled) Signal
add (disabled) Signal

With Whisper Systems making Signal work without Google Play Services and
Copperhead's Google-free fork, Signal is finally coming quite close to
being ready for inclusion in F-Droid.  I'm adding it is here to make it
easy to work together on making it build reproducibly.

f-droid/fdroid-data,f-droid/fdroiddata,f-droid/fdroiddata