WL-3119 Calendar invites render inline

To coerce email client to behave better, we make some adjustments to the
headers. Namely we use:

1. `multipart/alternative` instead of `multipart/mixed` in the top headers
   - this indicates that ical is an alternative rendering of the main email
2. Removes the `Content-Disposition` header
   - this is normally set to `attachment`, which is incorrect
3. Sets the `Content-Type` to `text/calendar`
   - because normally Java doesn't know what the mime type for ics is
4. Adds `method=request` to the `Content-Type`
   - the ical method in question is `REQUEST` so let's make this clear