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>

README.md

@@ -28,7 +28,7 @@ distribute traffic more evenly across all instances and avoid performance
 bottlenecks and rate-limiting.
 
 Farside also integrates smoothly with basic redirector extensions in most
-browsers. For an simple example setup, 
+browsers. For an simple example setup,
 [refer to the wiki](https://github.com/benbusby/farside/wiki/Browser-Extension).
 
 ## Demo
@@ -126,8 +126,6 @@ Farside's routing is very minimal, with only the following routes:
 
 - `/`
   - The app home page, displaying all live instances for every service
-- `/ping`
-  - A passthrough "ping" to redis to ensure both app and redis are working
 - `/:service/*glob`
   - The main endpoint for redirecting a user to a working instance of a
     particular service with the specified path
@@ -147,8 +145,8 @@ Farside's routing is very minimal, with only the following routes:
   - *Note: Uses Javascript to preserve the page in history*
 
 When a service is requested with the `/:service/...` endpoint, Farside requests
-the list of working instances from Redis and returns a random one from the list
-and adds that instance as a new entry in Redis to remove from subsequent
+the list of working instances from the db and returns a random one from the list
+and adds that instance as a new entry in the db to remove from subsequent
 requests for that service. For example:
 
 A user navigates to `/nitter` and is redirected to `nitter.net`. The next user
@@ -178,12 +176,10 @@ that their mission to centralize the entire web behind their service ultimately
 goes against what Farside is trying to solve. Use at your own discretion.
 
 ## Development
-- Install [redis](https://redis.io)
 - Install [elixir](https://elixir-lang.org/install.html)
 - (on Debian systems) Install [erlang-dev](https://packages.debian.org/sid/erlang-dev)
-- Start redis: `redis-server`
 - Install dependencies: `mix deps.get`
-- Initialize redis contents: `mix run -e Farside.Instances.sync`
+- Initialize db contents: `mix run -e Farside.Instances.sync`
 - Run Farside: `mix run --no-halt`
   - Uses localhost:4001
 
@@ -203,8 +199,8 @@ goes against what Farside is trying to solve. Use at your own discretion.
         <td>The port to run Farside on (default: `4001`)</td>
     </tr>
     <tr>
-        <td>FARSIDE_REDIS_PORT</td>
-        <td>The Redis server port to use (default: `6379`, same as the default for Redis)</td>
+        <td>FARSIDE_DATA_DIR</td>
+        <td>The path to the directory to use for storing instance data (default: `/tmp`)</td>
     </tr>
     <tr>
         <td>FARSIDE_SERVICES_JSON</td>