[#7717] Send email claim notification by email
[#7717] Allow claiming same email addresses
[#7717] Send claim notification if the email was confirmed by other user
[#7717] Limit the number of emails a user can claim
[#7717] PEP8 format & import optimization
[#7717] PEP8 format & import optimization
[#7717] Forbid verification emails for confirmed emails
[#7717] Invalidate existing verification emails