Use `Calendar invite email template` column in `Event settings`

When working on issue https://github.com/Azure/azure-sdk-tools/issues/3626 I found out that the email templates are being hardcoded inside the `onStart` method of each application.

If an admin would like to change the template, the only way to do it right now is to modify the code in edit mode. This is something we don't want.

Start by locating the Event settings model or UI and the applications whose email templates are hardcoded in their onStart methods. Trace how the “Calendar invite email template” column is stored and read; done means admins can change that setting without editing code and calendar invites use the configured template.

Written by the indexing model from the issue text.