Added percy integration

[yohanmishkin]

Percy is a visual testing tool designed to help catch regressions and reduce the amount of manual testing done in your application.

With Percy's recent introduction of a free plan and some important CSS changes planned for Ember Observer, Percy now makes a lot of sense.

This PR:

• installs ember-percy, an ember addon used to easily integrate Percy into ember projects
• adds percy snapshots to some of the most commonly accessed pages of the application

Notes

• ember-percy repo is here.
• PERCY_TOKEN has already been added to the CI env so snapshots for master will hopefully just begin being taken once the PR merges.

[codecov-io]

Codecov Report

Merging #143 into master will increase coverage by 3.34%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage    75.8%   79.15%   +3.34%     
==========================================
  Files         102      102              
  Lines         806      806              
==========================================
+ Hits          611      638      +27     
+ Misses        195      168      -27

Impacted Files	Coverage Δ
app/components/test-result-status.js	57.14% <0%> (+57.14%)	:arrow_up:
app/helpers/test-result-status-class.js	60% <0%> (+60%)	:arrow_up:
app/controllers/canary-test-results/date.js	75% <0%> (+75%)	:arrow_up:
app/components/canary-test-result-summary.js	100% <0%> (+80%)	:arrow_up:
app/routes/canary-test-results/index.js	100% <0%> (+100%)	:arrow_up:
app/routes/canary-test-results/date.js	100% <0%> (+100%)	:arrow_up:
app/routes/lists/top-addons.js	100% <0%> (+100%)	:arrow_up:
app/routes/addons/correct.js	100% <0%> (+100%)	:arrow_up:

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[yohanmishkin]

The naming convention I used for Percy snapshots feels a bit arbitrary and the placement of these new snapshots was a general good faith attempt. Please feel free to let me know if there's some other routes that are deserving of snapshots or if there's a better way to name them!

[yohanmishkin]

Sounds great, thanks @kategengler !

[yohanmishkin]

@kategengler I think this may be ready for a re-review. Let me know if I missed anything!

[kategengler]

Thanks for this!!!