refactor ltc-workspace-image to use `accetto/ubuntu-vnc-xfce-g3`

[capsulecorplab]

refactor ltc-workspace-image to use https://github.com/accetto/ubuntu-vnc-xfce-g3 as its base image, in the interest of minimizing image size, as well as enabling keybindings that are otherwise inaccessible via kasm

[capsulecorplab]

@TristanDamron seeking any guidance/support in this effort 🙂

[TristanDamron]

Looks like it'll be pretty easy to port this project to use the new image--All we need to do is change the base image in the Dockerfile and then change all of the Kasm references to something more generic (i.e. kasm_user is now persistence_layer)