User shall be able to change his/her mail adress by REST API

[de-jcup]

Situation

Currently only an administrator is able to change the user mail address

Wanted

A user shall be able to change his/her email address via REST.
The process shall ensure that the user has access to the mail address.

Solution

The process shall be similar to the registration process

1. A mail is sent to user to the old mail address.
2. Inside the mail there is a link with a one time token (valid for 10-24 hours)
3. The link is a deep link to a hidden Web UI Page
4. When the user opens the link he will shown two input fields (both for the email)
5. After entering the new email address confirming the email is changed
6. The user is rerouted to the login page from where he should be able to login with his new email

[de-jcup]

The process is related to #2374 - the rules shall be applied to user mail address changes from user side but also from administrator side!