Fixes to the system tests for event creation

Summary:
I made the events system tests pass for EAS (at least for the syncback operations we support on EAS -- namely creating and updating events)

Also, I fixed to additional auth bugs:
- we wouldn't take into account the Office365 MX servers when detecting providers
- we matched mx domains using `re.match` which matches only at the beginning of the line. I swapped this with `re.search` which gets us the behaviour we want (e.g: we can match inboxapp.mail.outlook.com using mail.outlook.com). Not sure if this would help with people getting unknown provider errors.

Test Plan: Ran the tests

Reviewers: emfree, kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D780