`hasGottenMail` is too strict

[Brent Roose]

There's currently a failing test, because our implementation of User::hasGottenMail is too strict. It only checks whether a specific mail has been sent, while actually it should (in some cases) take context into account.

For example: the NewRfcMail should be sent for every RFC.

My proposal is to do the following: add an interface HasMailId that mailables can implement, it has one method getMailId(): string that returns a unique id for this mail. For simple mails, we can use self::class, but for more complex mails, we can add the RFC id to it as well. This mailable ID should be saved in checked for in the mail_type column.

[Brent Roose]

It's fixed :)