Implement overridable index actions

Created by: anhari

Fixes #320 (closed).

• Replaces the use of locals with instance variables.
• Replaces .capture_view_locals in the ControllerHelpers module with a .value_assigned_to method which grabs the value assigned to an instance variable without requiring the rails-controller-testing gem.
• Updates views to use newly implemented instance variables, and replaces comments referencing local variables with ones referencing instance variables.
• Moves the logic behind show_search_bar and search_term into the Page::Collection object.
• Alphabetizes the Gemfile.