Frontend module cleanup

[paul-m]

Mainly updates to dkan_js_frontend.

• Turns route names like 'about' into dkan_js_frontend.about, etc. Try drush route | grep dkan
• Refactors the controller render method into less code, using values Drupal has already computed.
• Accepts the typehinted route matcher in the controller method so we can more easily target dataset routes for ID check.
• Adds cache invalidation information for the rendered page.
• Adds 'view content' permission to all our routes.

QA Steps

• Add manual QA steps in checklist format for a reviewer to perform. Be as specific as possible, provide examples if appropriate.

Checklist before requesting review

If any of these are left unchecked, please provide an explanation

• I have updated or added tests to cover my code
• I have updated or added documentation