WIP: adds migration and respective validation and update of limit of memory per project

This MR is related to the proposal in https://gitlab.com/gitlab-org/gitlab-ee/issues/559

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

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