Move EE code out of app/helpers and into ee/app/helpers

What does this MR do?

This moves all EE specific code out of app/helpers and into ee/app/helpers, with the exception of the necessary prepend lines to make this work.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8604

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer

TODO

• app/helpers/dropdowns_helper.rb
• app/helpers/button_helper.rb
• app/helpers/kerberos_spnego_helper.rb
• app/helpers/application_settings_helper.rb
• app/helpers/selects_helper.rb
• app/helpers/application_helper.rb
• Merge https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23631, then rebase
• Merge https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23632 / https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8735, then rebase