Refactor code to allow multiple issue boards management for projects
in CE

- Add serializers and Grape::Entity
- Replace to_json
- Add specs

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.

And use .rubocop.yml to exclude paths we don't care,
rather than using the cop itself to exclude.