Merge branch 'missing-raw-snippet-access-spec' into 'master'

Add missing security specs for raw snippet access

## What does this MR do?
It extends the project snippets access security specs to cover raw snippet paths as well.

When I was researching snippets for !7256, I noticed that specs existed for the HTML show view of project snippets but not the raw view. Seeing as this is a spec that is checking for access regressions on places where sensitive information might be kept, I thought it would be a good idea to cover the raw snippets access too.

To balance out the karma of adding in extra tests I also changed the tests to all use an `empty_project` spec.

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

With the aim of making the specs easier to read, I restructured some of them to use context blocks for each type of snippet.

I've used the same access rights defined for the show snippet paths for the raw snippet access. 

## Why was this MR needed?

To catch security regressions on raw snippet access for projects.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [-] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [-] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [-] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7300

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