Remove Redis dep, replace w/ native Elixir lib

This removes the dependency on Redis for core app functionality. Rather
than using the key/value store provided by Redis, Farside now uses a
key/val store provided by [cubdb](https://github.com/lucaong/cubdb) for
identical functionality but without reliance on a non-Elixir service.

This solution was chosen instead of ets, because storing instance data
in memory leads to a period of broken functionality whenever the app
restarts and hasn't re-populated instance data yet. It was also chosen
instead of dets, because the documentation for dets was pretty hard to
understand at first glance.

Tests and the CI build were updated to reflect the removed dependency on
Redis.

New environment variable `FARSIDE_DATA_DIR` can be used to point to a
directory where the instance data can be stored by cubdb.

Co-authored-by: Jason Clark <mithereal@gmail.com>

.gitignore

@@ -15,3 +15,4 @@ erl_crash.dump
 *.rdb
 .idea/
 *.iml
+*.cub