Add tests for label prioritization
user
```ruby
require 'spec_helper'

feature 'Prioritize labels', feature: true do
  include WaitForAjax

  let(:user)    { create(:user) }
  let(:project) { create(:project, name: 'test', namespace: user.namespace) }

  scenario 'user can prioritize a label', js: true do
    bug     = create(:label, title: 'bug')
    wontfix = create(:label, title: 'wontfix')

    project.labels << bug
    project.labels << wontfix

    login_as user
    visit namespace_project_labels_path(project.namespace, project)

    expect(page).to have_content('No prioritized labels yet')

    page.within('.other-labels') do
      first('.js-toggle-priority').click
      wait_for_ajax
      expect(page).not_to have_content('bug')
    end

    page.within('.prioritized-labels') do
      expect(page).not_to have_content('No prioritized labels yet')
      expect(page).to have_content('bug')
    end
  end

  scenario 'user can unprioritize a label', js: true do
    bug     = create(:label, title: 'bug', priority: 1)
    wontfix = create(:label, title: 'wontfix')

    project.labels << bug
    project.labels << wontfix

    login_as user
    visit namespace_project_labels_path(project.namespace, project)

    expect(page).to have_content('bug')

    page.within('.prioritized-labels') do
      first('.js-toggle-priority').click
      wait_for_ajax
      expect(page).not_to have_content('bug')
    end

    page.within('.other-labels') do
      expect(page).to have_content('bug')
      expect(page).to have_content('wontfix')
    end
  end

  scenario 'user can sort prioritized labels', js: true do
    bug     = create(:label, title: 'bug', priority: 1)
    wontfix = create(:label, title: 'wontfix', priority: 2)

    project.labels << bug
    project.labels << wontfix

    login_as user
    visit namespace_project_labels_path(project.namespace, project)

    expect(page).to have_content 'bug'
    expect(page).to have_content 'wontfix'

    # Sort labels
    find("#label_#{bug.id}").drag_to find("#label_#{wontfix.id}")

    page.within('.prioritized-labels') do
      expect(first('li')).to have_content('wontfix')
      expect(page.all('li').last).to have_content('bug')
    end
  end
end
```
assistant