Add meeting invite into emails sent to activity assignees

Attach a meeting invite .ics file to emails sent to activity assignees. This will make it easier to import into their calendars. It shouldn't break any existing behavior because if they don't want to import it they can just ignore the attachment and read it like a regular email same as now.

Out of scope:

• Tracking accept/deny replies
• Meeting updates or cancellations

Technical note:
The code that sends out the emails is in two places (regular activity and case activity) so need to update both.