Purpose of this commit is to correctly extract email address from the email
field of models inheriting from the blacklist mixin. Indeed email field
could contain a formatted address like "Raoul Grosbedon <RAOUL@example.com>".
Related blacklist entry would be raoul@example.com. We have to extract
the email from the email field and lowerize it in the SQL query or the
computation in order to have a fully working blacklist mixin. This fixes
some bits left at commit 2ff9b379ef.
This commit is related to task ID 33224 (original blacklist implementation
done for v12) and its PR #25966 as well as task ID 1889703 (tests and fixes)
and its PR #27330. Done with collaboration of @dbeguin.