Motivation
At the moment in order to send emails, static class UserMail provides a bundle of static functions and helpers. This makes testing impossible.
class Mailer service was introduced as part of T247229 but it just calls static method and didn't change calls
Expected result.
- UserMail class and all its methods should be marked as deprecated
- Calls to UserMail's methods should be changed to retrospective methods of Mailer
- Mailer should be injected through DI where it's possible or MediaWikiServices::instance() where DI is not introduced.