As an Org Admin, Org Manager, or Unit Manager, I want to configure a default `message` and/or `sender_name` at the Org and/or Unit level to be used for all invitations sent for my Org and/or Unit.

Use Case

Org Admins, Org Managers, Unit Managers, and CO Admins all have the ability to invite users to join collaborations.
Invitations may be sent via the SRAM GUI or via the Organisation API
The SRAM GUI provides a text input field which allows the sender to specify an optional message to be included with the invitation. The SRAM GUI does not currently provide a text input field to specify a sender_name
The SRAM Org API provides the user with the opportunity to specify a message and a sender_name to be sent with the invitation.
For administrators who send many invitations, it would be helpful to be able to set a configuration parameter at the Org and/or Unit level that provides a default message and sender_name for invitations

The defaults are displayed to someone using the SRAM GUI to send an invitation and they have the ability to override the defaults per-invitation.
The defaults can be configured at the Org Level and Unit Level and are scoped appropriately for COs within the Org and Units
The defaults are applied (unless they are overridden at sending time) to invitations sent using the SRAM GUI and the Org API

Default message/sender_name Configuration can be added to the Org Configuration UI.
Alternatively, we may consider (along with Edwin) if we should do something different in the UI to handle this and other Unit-inclusive configuration parameters, see also #1688