[JENKINS-19433] - Added handling of user-defined e-mail addresses in FastEmailResolver.

This change adds a new routine for fast email resolution.
If a user has defined his own e-mail on the profile page, it will be resolved in the Mailer plugin with a highest priority.

Resolves https://issues.jenkins-ci.org/browse/JENKINS-19433

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>