Add visible projects API

What does this MR do?

Add a new /projects/visible API endpoint. Originally created by @ben.boeckel in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5970.

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

Does the API make sense?

Why was this MR needed?

The /projects endpoint only returned projects the user was explicitly a member of.

What are the relevant issue numbers?

Closes #19361 (closed), #3119 (closed).

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
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together