mailer module refactor

[javorszky]

As #2742 painfully reminded us, the mailer module needs a boatload of refactor. The current implementation has circular dependency between mailer and the api.

The module should only concern itself with sending mail as per given data, and returning a status depending on whether sending it was successful or not. API calls and settings readings should be done earlier.

@ErisDS, can you assign this to me, please? I have a few ideas of where to move stuff*.

*subject to all other ideas, obviously