Move 'Explore Snippets' Spinach feature to Rspec

## What does this MR do?
It moves the `features/snippets/discover.feature` Spinach test to a Rspec feature.

## Are there points in the code the reviewer needs to double check?
The original feature was called 'Discover Snippets', but the UI no longer reflects this wording. The new Rspec feature is called 'Explore Snippets' to reflect UI/Controller/View naming in use.

## Why was this MR needed?
As part of deprecating the Spinach test suite.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [-] [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)
- [-] 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?
#23036

See merge request !7349