Add to calendar link in email is not time zone aware

[kdeal]

If a user clicks on the link then it will create the event in their timezone and not in the timezone the event is in. This can lead to the event being scheduled at the wrong time.

[ymilki]

Turns out, an old TODO was made for this issue: https://github.com/Yelp/beans/blob/fda948f38f4ce2bc789f4c4bfc63b3f40fbf302d/api/yelp_beans/send_email.py#L174

Given now that workforces are now scattered across the globe, this is an important quality of life bug to fix.

This is similar to #141, but we can probably take advantage of a user's google calendar timezone.

[kdeal]

I wonder if this would be a good time to convert to using an ics file instead of a link.