Me: Settings: validate email address before letting user submit & show better errors

[v18]

When changing the email address on me/account, then email field does not have any validation. Try entering a nonsensical email address like 'email' and you see this error message:

Instead we should do these two things:
(1) Display a more specific error message
(2) Validate the email address before the user saves the settings

[v18]

Closed in favour of #975