CHORE: Apple Touch Icons 📱

At present when we visit the app http://localhost:4000 in **_Desktop_ Safari** we see the following error:

```sh
[info] GET /
[debug] Processing with AppWeb.PageController.index/2
Parameters: %{}
Pipelines: [:browser]
[info] Sent 200 in 89ms
[info] GET /apple-touch-icon-precomposed.png
[debug] ** (Phoenix.Router.NoRouteError) no route found for GET /apple-touch-icon-precomposed.png (AppWeb.Router)
(app 0.1.0) lib/phoenix/router.ex:406: AppWeb.Router.call/2
(app 0.1.0) lib/app_web/endpoint.ex:1: AppWeb.Endpoint.plug_builder_call/2
(app 0.1.0) lib/plug/debugger.ex:136: AppWeb.Endpoint."call (overridable 3)"/2
(app 0.1.0) lib/app_web/endpoint.ex:1: AppWeb.Endpoint.call/2
(phoenix 1.6.6) lib/phoenix/endpoint/cowboy2_handler.ex:54: Phoenix.Endpoint.Cowboy2Handler.init/4
(cowboy 2.9.0) /deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
(cowboy 2.9.0) /deps/cowboy/src/cowboy_stream_h.erl:306: :cowboy_stream_h.execute/3
(cowboy 2.9.0) /deps/cowboy/src/cowboy_stream_h.erl:295: :cowboy_stream_h.request_process/3
(stdlib 3.15.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

[info] GET /apple-touch-icon.png
[debug] ** (Phoenix.Router.NoRouteError) no route found for GET /apple-touch-icon.png (AppWeb.Router)
(app 0.1.0) lib/phoenix/router.ex:406: AppWeb.Router.call/2
(app 0.1.0) lib/app_web/endpoint.ex:1: AppWeb.Endpoint.plug_builder_call/2
(app 0.1.0) lib/plug/debugger.ex:136: AppWeb.Endpoint."call (overridable 3)"/2
(app 0.1.0) lib/app_web/endpoint.ex:1: AppWeb.Endpoint.call/2
(phoenix 1.6.6) lib/phoenix/endpoint/cowboy2_handler.ex:54: Phoenix.Endpoint.Cowboy2Handler.init/4
(cowboy 2.9.0) /deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
(cowboy 2.9.0) /deps/cowboy/src/cowboy_stream_h.erl:306: :cowboy_stream_h.execute/3
(cowboy 2.9.0) /deps/cowboy/src/cowboy_stream_h.erl:295: :cowboy_stream_h.request_process/3
(stdlib 3.15.2) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
```

No doubt there will be a similar error when visiting the Heroku version on **_Mobile_ Safari** ... 📱
Ideally we want decent touch icons so that:
a. The Favicon appears when using the app on Desktop (rare)
b. The Touch ("App") Icon appears when we create a Shortcut to the Web App (Heroku) on iOS.
c. No annoying errors in the logs which are horrible UX for people running the app on their `localhost`

# Todo

+ [ ] Check Heroku Logs once the MVP App has been deployed #53 🚧
+ [ ] Can be done _after_ #57 has been reviewed/merged. ⏳
+ [ ] Add any necessary files in `priv/static/images` so that Phoenix _finds_ them. 📂
+ [ ] Re-test on both Desktop & Mobile ♻️