To make sure all known issues are linked to the correct epic, I've gone
through the code base, and updated the comments where required.

Enables frozen string for the following:

* app/controllers/*.rb
* app/controllers/admin/**/*.rb
* app/controllers/boards/**/*.rb
* app/controllers/ci/**/*.rb
* app/controllers/concerns/**/*.rb

Partially addresses #47424.

This reverts merge request !16968

Signed-off-by: Rémy Coutable <remy@rymai.me>

Fixes #14585.

Closes #2662

- 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.

This controller is now the target for `root_url`. It sub-classes
DashboardController so we can render the old default without a redirect
if the user hasn't customized their dashboard location.