This version apparently has less issues with the graphics display, i.e.
it uses a slightly newer version of Phaser.

I put it in a seperate branch for now; in the long term, it may be better
to just provide several "flavours" of workadventure in the main branch.

Previously, it still assumed that there was an ./overlay.nix.

Since overlays in general appear to be slightly broken and generate
an unwieldy amount of infinite recursion, this is no longer the case
and test.nix uses a (slightly uglier) method for testing instead.

(it refused to evaluate on another server for an unknown reason,
and instead led to infinite recursion)

Maximal sharing means that even if the workadventure sources are imported
in all nix files, there will still only be one copy of it in the nix store
--- nevertheless, it seems reasonable to only have them once in the code,
too, if only to make upgrading them easier.

Since I change things frequently rn and often only very specific
commits of both repositories will work together at all, it
doesn't seem logical to have them in separate repos any more.