Support exporting data for the detailed index.

This adds support to the rummager_export script for exporting data for
the detailed index.  Add a note on how to do this in the README.  Change
the README to suggest dumping the data for loading into the index to a
temporary file rather than piping the output of the export straight to
the bulk load script, because the export script tends to use a lot of
memory and is liable to fall over if other processes in the VM are
trying to do stuff.
Uniqueify classes_to_index in rummager export.

Without this, some formats get dumped to the export file multiple times.
Fix commands suggested for running bulk export
Uniqify class lists in whitehall.
Merge pull request #1272 from alphagov/supported_detailed_index_in_rummager_export

Support exporting data for the detailed index.