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.

Make it easier to override the set of loggers, by putting them in a
separate method.

Also modify spec so it tests for links, instead of checking for
regular text.

We should be defining the list of loggers in the controller, not the
view.

Handles stuff that's shared across admin controllers.