WIP: Add sunspot-rails for heterogeneous searching of materials.

Relates to https://www.pivotaltracker.com/story/show/56305416
see: https://github.com/sunspot/sunspot/blob/master/README.md
move sunspot_solar to dev & testing gem sets
Don't re-run all the specs when a helper is changed.
Some documentation for sunspot.
Change searchable attributes in Investigation and Activity for Sunspot
Add Spec Helpers for unit testing Search models.
Search model.  This class encapsulates all our Material searching logic.
searchable blocks for ExternalActivity and ResourcePage

Part of the Solr overhaul
Using material_type to group search results
Return 'ExternalActivity' as material_type when no template
white-space cleanup.
1ˢᵗ pass on cleaning up search_controller.rb (tests failing, &etc.)
1ˢᵗ pass simplification of _filters.html.haml
Add material_type to Investigation search fields.
Search includes ExternalActivity & Resource Page. Use material_types.
search.rb  :order renamed to :sort_order.
Normalizing searchable interface on models
Blocks for nils in search interface definitions
Other search-interface constant blocks
Defining domain and grade_span for activity search