Use Rails helper method to generate grouped select

Perhaps this was built manually as the `with_translations_for` method was
involved? It doesn't appear to have mattered, as including translations
in the AREL code seems to have done the job nicely; the HTML produced is
identical, other than for the order of the optgroup tags (which come out
in alphabetical order with this new approach).
WIP: Select document series and group when speed tagging

I've not finished this, but we're getting there. Things to do before
amending this commit:

- Update Edition::HasDocumentSeries, defining `document_series_group_ids=`
  instead of `document_series_ids=`.
- Fix N+1 performance problem introduced to the speed tagging page (not
  sure if it requires `:inverse_of` or just a more complicated hash for
  `#includes` when listing organisations). The `series_and_group_name`
  method in the helper is causing new queries to get fired off when
  de-referencing a group's document series.
- Make sure that the whole feature works; there's a broken unit test,
  but no cucumber coverage (which may be totally fine).