Analyze the specialist_sectors field

We were originally not analyzing the specialist_sectors field because we
had a plan to move to storing this in an internal tags field.  However,
we don't currently have any searches which require a single internal
tags field, so this is probably a misstep for now.  Changing the field
to be analyzed will make it available for searches after we next run a
deploy:migrations.

Also, don't allow use of the tags field in searches, since it shouldn't
be visible externally.
Expand specialist_sectors field

The specialist sectors aren't yet being indexed, but when they are this
will the specialist_sectors field in search results.

The main reason for implementing this now is that we also want to store
example values in the SpecialistSectorsRegistry, so we need to create
that object first.
Support stubbing the metasearch index

Also, memoize the index stubs so that repeated calls to them within a
test return the same stub.
Functional test for sector expansion
Merge pull request #252 from alphagov/expand_specialist_sectors

Expand specialist sectors

https://www.pivotaltracker.com/story/show/73972778