Revert outcome_type to being of type identifier
Revert outcome_type to being of type identifier

Specialist Publisher is sending a value of `[""]` for this field when
the document doesn't have an `outcome_type`. When this field was an
`identifier` (singular), Rummager did not index a value for this field.
Now this field is `identifiers` (plural), Rummager is indexing the value
as is. This causes Rummager to return `[null]` when Finder Frontend
requests the results, which causes Finder Frontend to error.

This issue should be addressed independently in all three applications,
as they're all acting in slightly surprising ways, but to give us time
to sort that out let's revert back to config that Just Works.

theodi/rummager,theodi/rummager,alphagov/rummager,alphagov/rummager