Currently, only admins can change the email address of a user (in the users table). Instead, each user should be able to change their own email.

The UI should be similar to the "change password" screen. Some options:

A) We create a new "User" page (available via the avatar at the top right) and move stuff there.
B) We add a new "Change Email" entry to the avatar menu.

B is probably a bit simpler for now and also sufficient.

Additionally, admins should not be able to change the email anymore. The email address in the users table should be read only.