Clean up overlap between dashboard and explore

- Clean up overlap between dashboard and explore. ... 5d785457
  - Split up SnippetsController into separate dashboard and explore sections.
  - Use consistent page titles, header titles and sidebars between dashboard and explore sections when signed in or not.
- Consistently case `Back to X` links. 260fcd45
- Link to help from signin page bottom. d92696d3
- Make Help accessible for guests. c258e977

cc @dzaporozhets

See merge request !1260