Migrate Google calendar bot.

[roberthoenig]

zulip/zulip#2771 is a PR for adding a Google Calendar bot.
The PR is big, and has potentially the most comments any PR in the Zulip project ever had.

We should:

• Copy the PR's code and create a new Google Calendar bot in this repo.
• Test the bot. If it works, we should just merge it. Then, people can collaborate on code-style issues, adding tests, etc.

[itrajkov]

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[itrajkov]

#198 Added it.