Add an indexed field to Institution to help indicate which ones need to be hidden in the Wayfinder

Hidden institutions are: 'test' and 'special' accounts

So like this

class Institution
  searchable do
    ...
    boolean :show_in_wayfinder
    ...
  end
  ...
  def show_in_wayfinder
    !(test_site || special)
  end
  ...
end