Add a system hook for when a project is updated.

What does this MR do?

Adds a system hook for when a project is updated.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

I needed a system hook for when a project is updated

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing - see my comment in https://github.com/gitlabhq/gitlabhq/pull/9955#issuecomment-238090254
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together