A user should be able to change mail

[naturalprogrammer]

As a user, I should be able to request changing my email, so that I can later confirm the change.

1. A user should be able to request changing email (POST /user/email-change-request {password, oldEmail, newEmail})
2. Upon success, he/she should get a mail with a confirmation link in his new email box
3. By clicking on the link, he/she should be able to go to the front end when this endpoint will be called: PATCH / user/email {token: a-je-token} and email will be changed.
4. Old login token should be invalidated upon email change.