SPIKE: Add PTO to external calendars

[meganvbarton]

For MVP we will want to integrate with google calendar.

Is this going to be possible? How do we do it?

[atomdmac]

Very simple proof-of-concept can be found here. Current features include:

• Allow user to link their Google account with their Slack account (maybe not necessary for our use case but demonstrates what the account linking/OAuth process looks like).
• List the next 10 events on the user's calendar

From looking at the GCal API docs, it appears that this POC could be fairly easily extended to include functionality for adding/editing/removing events, providing alerts based on calendar events, etc.