Parameterize mailer attributes

[arkirchner]

The new way of giving parameters to mailer is with the set method. This offers some benefits in testing. All mailers should be migrated to follow this pattern.

[arkirchner]

Not relevant.

[MrSerth]

Not relevant.

Why do you think this issue is no longer relevant (and close it as completed the same time)? @nenock Didn't we agree to migrate all mailers? Can you help out, please?

[MrSerth]

Given the two PRs #2999 and openHPI/codeharbor#2061, this ticket has been tackled and hence the status "closed as completed" is finally deserved.