Move `subscribed?` out of the Epic show request cycle

Currently the two most expensive queries when executing the EpicsController#show action in HTML format (i.e. simply loading the initial HTML page for an epic) are called because we include subscribed in the data to be bootstrapped from the page.

This adds between 50-150ms to page load times, sometimes more. This is because all notes need to be loaded for the epic and checked.

I believe the purpose of this is to check if the current user is subscribed to the epic by virtue of having participated in the discussion.

I experimented with removing this by simply commenting out https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/presenters/epic_presenter.rb#L48 and didn't identify adverse effects.

If these queries cannot be removed entirely we should review the queries to see if they can be consolidated or improved.