Make namespace API available to all users

### What does this MR do?

This MR makes it possible for a user to query namespaces to which he/she has access. Also, it adds documentation for the existing API.

### Why was this MR needed?

Even though the `groups` API exists, it might still be useful to have an endpoint that tells the namespace type (e.g. `user` vs. `group`), especially if a user has access to a number of different projects.

### What are the relevant issue numbers?

Closes https://github.com/gitlabhq/gitlabhq/issues/9328

See merge request !708