mods indexing: index qualified dates and end dates in their own fields (...

...previously some of these dates weren't being indexed at all
Good stuff.

This series of if statements could be condensed.
Ex:
DATE_QUALIFIERS = ["approximate", "inferred", "quesitonable"]
if qualifier in DATE_QUALIFIERS:
  data = self._add_or_extend(data, 'mods_%s_%s_ssim' % (date_name, qualifier) , [date.text])