Add support for attaching `.ICS` files through the API

[andrewleith]

Description

As a user, I need to be able to attach .ics files to my email notifications to make it easier for recipients to save appointments, etc. into their calendars.

WHY are we building?

• To make it easier for end users to add events to their calendars with a click.

WHAT are we building?

• Ability to attach .ics files to email notifications

VALUE created by our solution

• Notification recipients can add events to their calendars with a single clic,

Acceptance Criteria

• [x] Security team has been consulted
• [x] .ics files are accepted when attaching a file to a notification

[YedidaZalik]

Andrew talked to Security and no issue Jimmy and Andrew would like to do right away because it's an easy fix and could just do for this particular service