(WIP)61 Implements google calendar integration backend

[AuraOfDivinity]

Problem
Github Issue Number: #61

Solution of problem

Implements the backend functionality of the google calendar integration

Type of Change

• Bug fix
• New Feature
• Development of UI/UX prototypes
• Small refactor
• Change in Documentation

Checklist

• My code follows the same style as the codebase
• My Code change requires a change in documentation
• I have updated the Readme accordingly
• I made PR against development branch
• I have run the test cases locally and it's passing.
• I have squashed my commits

[codecov]

Codecov Report

Merging #88 into development will decrease coverage by 2.50%.
The diff coverage is 33.33%.

@@ Coverage Diff @@ ## development #88 +/- ## =============================================== - Coverage 55.07% 52.57% -2.51%  =============================================== Files 16 18 +2 Lines 345 350 +5 Branches 41 39 -2 =============================================== - Hits 190 184 -6  - Misses 131 141 +10  - Partials 24 25 +1 

Impacted Files	Coverage Δ
app/routes/calendar.js	14.28% <14.28%> (ø)
app/utils/token-utils.js	40.00% <40.00%> (ø)
app/routes/oauth.js	75.00% <75.00%> (ø)
app.js	88.09% <90.90%> (+8.09%)	⬆️
app/models/GoogleUser.js	100.00% <100.00%> (ø)
app/models/Post.js
app/models/Comment.js
... and 4 more

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