User mapping - send reassign request email

After the group owner selects the user for the reassignment, an email should be sent to the user. This will allow the user to approve or reject the reassignment.

In this issue, we will implement both backend functionality to send the email to the user as well as email design

For your reference, you can find the design of the email here.

The "Review reassignment details" button should send the user to the show action implemented in #461192 (closed)