Add AppImage built target for Linux (#2864)
Add AppImage built target for Linux (#2864)

This hooge PR gets `yarn package-linux` to create an AppImage file which will 
allow GNOME users to avoid having to launch Decrediton via CLI.
Closes https://github.com/decred/decrediton/issues/2277
decred/decrediton,decred/decrediton,decred/decrediton,decred/decrediton,decred/decrediton,decred/decrediton