PR: Add /ping endpoint and image to Wake Heroku Dyno

[nelsonic]

This PR relates to dwyl/ping#1

[nelsonic]

@SimonLab hopefully the docs in https://github.com/dwyl/ping are clear enough that the purpose of this PR is obvious. (but if not, please LMK!)

The way to test this is working is to view the README.md of this branch: https://github.com/dwyl/phoenix-content-negotiation-tutorial/tree/add-ping-to-wake-heroku-dyno

And then visit the Heroku app: https://phoenix-content-negotiation.herokuapp.com
It should load instantly rather than having to wait for the dyno to wake.
(because the app was "woken up" by loading the README.md in the browser ...)

Thanks for reviewing/merging. 👍

[codecov]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         5    +1     
  Lines           11        13    +2     
=========================================
+ Hits            11        13    +2     

Impacted Files	Coverage Δ
lib/app_web/controllers/ping_controller.ex	100.00% <100.00%> (ø)
lib/app_web/router.ex	100.00% <100.00%> (ø)

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[SimonLab]

Looks good 👍