Revert "Skip the unneeded current_search_session filter. Fixes #423"

This reverts commit 947c289575c07aad5d2a890a0c53619394370c74.
fix current_page_context from the home page; fixes #424
add integration tests for blacklight search context from browse and feature pages
give test searches an exhibit context
use before_filter :except, instead of custom logic in the filter
refactor exhibit mixin loading so that: spotlight::controller provides blacklight_config and search_action_url overrides, spotlight-overrides of blacklight mixins include the blacklight mixin they override, and provide an exhibit_specific_blacklight_config method
update cancan to cancancan
any user can read any published page or search
allow friendly_id'd associations of exhibit to use friendly id accessors
update load_resources to not use shallow routing for scoped resources
add friendly_id :history feature to pages and searches
update shared views to not require a current exhibit to render correctly.
searches should use their exhibit blacklight config, not the global one.
update Spotlight::Controller to make an exhibit context optional
update report a problem spec to use an exhibit context
update home pages controller to use a spec-specific exhibit
Use cancan to load exhibits for all controllers that use an exhibit context
Merge pull request #436 from sul-dlss/page-context

Page context