Allow public access to some Project API endpoints

## What does this MR do?

This opens up a few endpoints in the Project API:

- `GET /projects/visible` (returns public projects only)
- `GET /projects/search/:query` (returns results only for public projects)
- `GET /projects/:id` (only if the project is public)
- `GET /projects/:id/events` (only if the project is public)
- `GET /projects/:id/users` (only if the project is public)

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

I've chosen to explicitly add `authenticate!` to GET methods that still need a current user.

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing

Part of #4269

See merge request !7843