Tweak organisation importer to use document IDs.

Unlike the slug, they're indexed, so we're not doing a collection scan
on each lookup.
Index organisation names.

We're often pulling these out using the `in_name_order` scope, so it's
sensible to maintain an index to avoid sorting them every time.
Index the organisation IDs on a need.

When we're querying the need API for all needs related to an
organisation, we shouldn't have to scan through all the needs to find
the ones we want.
Order by Mongo ID rather than the `need_id` field.

We're declaring these to be the same thing when we create the need, so
there shouldn't be any functional difference, but the internal document
ID is automatically indexed by MongoDB.
Merge pull request #34 from alphagov/indexing

Indexing