Add additional user and email services

Refactors the current code in CE to use services to update a User or an Email. It will unify changes to the entities and make it easier to add audit events to them - part of https://gitlab.com/gitlab-org/gitlab-ee/issues/1370

Instead of backporting from EE to CE, we can work on the common ground here and add the extra audit stuff later on an EE MR.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together