[Feature] Add AppImage Support

[asddsaz]

Feature: Create AppImage for LBRY

Why? Many reasons, the main being that it is portable and easy to use.

[tzarebczan]

Thanks for opening this issue!

Please see previous discussions about appimage in #953
#332

For now we are sticking with deb files as it supports a majority of Linux users and for the reasons discussed in the issue above. We also provide flatpak support which covers other non-debian distros: https://github.com/lbryio/lbry-desktop/blob/master/README.md#install. Linux users make up a small portion of our user base, so I don't think we'll be dedicated time to this anytime soon, will close for now.

Also, before opening issues, please do a search on github for previous discussions on the topic, thanks for understanding!