Make the `relatable_items` query use an index.

Without these changes, the `relatable_items` method produces the list
using a BasicCursor, which is slow and has a limit on the site of data
set it'll work on. Adding a suitable index, and tweaking the query so it
can use it, avoids this problem.

Restricting the fields returned to the name only means the result can be
retrieved solely from the index, without having to deal with the full
documents at all. Since this is all we display in Panopticon, it's worth
omitting everything else.
Merge pull request #101 from alphagov/relatable-items-index

Make the `relatable_items` query use an index.