Reduce code included by RepositorySizeError
We currently include ActionView::Helpers in RepositorySizeError: https://gitlab.com/gitlab-org/gitlab-ee/blob/0bb9b964833cd96ce74c2fb3167dc7a8b4bda110/lib/gitlab/repository_size_error.rb#L3
But the only thing we seem to use from it is number_to_human_size, so this is overkill. We should only include ActiveSupport::NumberHelper, which contains the helper.
cc @patricio