Add a config file for whitehall's solr index

Whitehall itself has a seperate solr path to the mainstream solr path.
Keep this consistent and obvious by using a different config file.
Add stubs for whitehall's SolrWrapper object

Create a stub for each of the SolrWrapper objects we create. Keep these
physically seperate so that we can still use the same API through our
code.

The DelSolr stubs were needed because you can't do fine-grained matchers
using Mocha.
Rename @solr instance to @mainstream_solr

As we're introducing another instance variable of the same type,
keep these two instances easily recognisable by providing some
information on their defining characteristics.

In this case, the respective mainstream and whitehall solr buckets.
Rename builer method to provide more context

As we're using @mainstream_solr as the instance method, update the
solr method to reflect this new name.
Hide the "Specialist guidance" filter when no results present

This is just a test to get started. It makes sure that we're not
showing the filter for specialist content when the solr bucket
doesn't have any search results.
Show specialist filters when specialist results exist

When there are specialist (whitehall) results in the solr index
returned, make sure to show the relevant filter on the results
page.
Specialist results should be included in total results count

When working out the final results count, make sure to include
the specialist results in this -- they're just as important!
Specialist filter has a count of the results

When there are specialist results present, we should also show a
count of the number of results we're showing.
Show specialist results after mainstream results

This is just a simple each block which runs through and does
something very similar to what the equivalent mainstream
Document printer does.

This might need some confirmation with the folks in whitehall as
to what's going on.
Merge master into specialist-results branch

This pulls in the work done as part of pull request #12pull request #12pull request #12
Rename mainstream_solr to primary_solr

The rummager app is used in two places at the moment: for mainstream
and for whitehall. Calling the main search object "mainstream_solr"
isn't fair to the whitehall team as their main search object will
use the whitehall index.

Instead, call the "main" SolrWrapper object the primary_solr
instance.
Rename whitehall_solr to secondary_solr

As the app is used in multiple places, this second index might not
actually be used to query whitehall's search index. Instead, it's
better to call it the secondary index to distinguish how it's
used and be defined by the YAML config file that gets deployed.
Add a feature flag for using secondary solr index

A feature flag is required to make sure that using a second index
isn't turned on by default. This is because the whitehall-search
also uses the same rummager codebase.
Have defaults for feature flags in all environments
Don't reference mainstream/specialist in test names

The tests are actually testing the primary/secondary solr instances.

This change makes it clearer that it's what the tests do.
Allow passing in the param_filter through config for secondary solr index
No need to have a preview block set

Our preview environment has the environment variable set to
'production', and 'preview' is never seen in our current setup.
Merge pull request #11 from alphagov/specialist-results

Show specialist results alongside mainstream results