Move 'Explore Snippets' Spinach feature to Rspec

This commit moves the `snippets/discover.feature` Spinach test to a
Rspec feature, as part of deprecating the Spinach test suite.

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.

- Remove Spinach discover snippets feature and steps
- Add Rspec feature test

features/snippets/discover.feature

-@snippets
-Feature: Snippets Discover
-  Background:
-    Given I sign in as a user
-    And I have public "Personal snippet one" snippet
-    And I have private "Personal snippet private" snippet
-    And I have internal "Personal snippet internal" snippet
-
-  Scenario: I should see snippets
-    Given I visit snippets page
-    Then I should see "Personal snippet one" in snippets
-    And I should see "Personal snippet internal" in snippets
-    And I should not see "Personal snippet private" in snippets

features/steps/snippets/discover.rb

-class Spinach::Features::SnippetsDiscover < Spinach::FeatureSteps
-  include SharedAuthentication
-  include SharedPaths
-  include SharedSnippet
-
-  step 'I should see "Personal snippet one" in snippets' do
-    expect(page).to have_content "Personal snippet one"
-  end
-
-  step 'I should see "Personal snippet internal" in snippets' do
-    expect(page).to have_content "Personal snippet internal"
-  end
-
-  step 'I should not see "Personal snippet private" in snippets' do
-    expect(page).not_to have_content "Personal snippet private"
-  end
-
-  def snippet
-    @snippet ||= PersonalSnippet.find_by!(title: "Personal snippet one")
-  end
-end

spec/features/snippets/explore_spec.rb

+require 'rails_helper'
+
+feature 'Explore Snippets', feature: true do
+  scenario 'User should see snippets that are not private' do
+    public_snippet = create(:personal_snippet, :public)
+    internal_snippet = create(:personal_snippet, :internal)
+    private_snippet = create(:personal_snippet, :private)
+
+    login_as create(:user)
+    visit explore_snippets_path
+
+    expect(page).to have_content(public_snippet.title)
+    expect(page).to have_content(internal_snippet.title)
+    expect(page).not_to have_content(private_snippet.title)
+  end
+end