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.

Closes #19361, #3119.

See merge request !6681