Google Code-in

In the main zulip/zulip repository, bots/gcal-bot is an integration that will send your Google calendar updates into Zulip.

It has some documentation in its --help output, but it isn't obvious to users how to use it. We should clean it up, test that it works, move it to api/integrations/ and then document it nicely on https://chat.zulip.org/integrations/, similar to how the Twitter integration is documented.