Limit recovery extended mails

https://gitlab.e.foundation/e/infra/backlog/-/issues/4072

It should raise an error when setting more than 5 accounts with this scheme: prefix+expansion@domain.tld

for example we have 5 accounts:

• user+mail1@blah.fr
• user+mail2@blah.fr
• user+mail3@blah.fr
• user+mail4@blah.fr
• user+mail5@blah.fr

then:

• user+mail6@blah.fr

shouldn't be able to register

but:

• user+mail1@blah.fr should be able to change to
• user+mail7@blah.fr