We had similar code in a few places to redirect to the last page if
the given page number is out of range. This unifies the handling in a
new controller concern and adds usage of it in all snippet listings.

- Avoid N+1 queries for authors and comment counts
- Avoid an additional snippet existence query

Enables frozen string for the following:

* app/controllers/dashboard/**/*.rb
* app/controllers/explore/**/*.rb
* app/controllers/google_api/**/*.rb
* app/controllers/groups/**/*.rb
* app/controllers/import/**/*.rb
* app/controllers/instance_statistics/**/*.rb
* app/controllers/ldap/**/*.rb
* app/controllers/oauth/**/*.rb
* app/controllers/profiles/**/*.rb

Partially addresses #47424.

Douwe Maan authored 7 years ago and Bob Van Landuyt :neckbeard: committed 7 years ago

Refactor snippets finder & dont return internal snippets for external users

See merge request !2094

- 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.