Add presence support

[sreecodeslayer]

There is a gotcha here:

Since there is no concept of user logins and sessions based on it, I felt its okay to mark a user as online once a message with a name is made. Presence would then track that particular user as opposed to tracking when the page loads.

Also, I've added a timestamp field to the Phoenix Presence tracker in the backend, but I chose to keep the UI simple.

Closes #14

[codecov]

Codecov Report

Merging #32 into master will decrease coverage by 3.57%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage     100%   96.42%   -3.58%     
==========================================
  Files           6        6              
  Lines          22       28       +6     
==========================================
+ Hits           22       27       +5     
- Misses          0        1       +1

Impacted Files	Coverage Δ
lib/chat/application.ex	100% <ø> (ø)	⬆️
lib/chat_web/channels/room_channel.ex	93.75% <85.71%> (-6.25%)	⬇️

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[nelsonic]

@sreecodeslayer We agree that Phoenix presence is a desirable feature to add. 🚀
However the purpose of this repository is to serve as a beginner tutorial for Phoenix.
As such we cannot accept a pull request without tests and documentation consistent with the existing example(s).
If you have time to add a couple of tests for the code, please do.
Thanks! 👍

[nelsonic]

https://github.com/dwyl/phoenix-liveview-chat-example#14-presence

[nelsonic]

Given that there is quite a divergence from this PR to the main branch
and even larger once #152 (Phoenix v1.7 update) is merged.
I'm reluctantly closing this. 😞
Would love to have this contribution re-created off main with tests. 🤞