set collaborations tab as active when page is accessed

fixes CNVS-17721

test plan:
- go to /plugins and enable a collaboration tool (i used etherpad with
  dummy settings for this test)
- go to a course
- if the collaborations tab doesn't show up, you might have to clear
  a cache, which can be done by going to the course settings, tab
  navigation, and hitting save
- click on the collaborations tab
- it should be highlighted as "active" in the tab list
- it should have a navigation crumb in the top bar

Change-Id: I9441ee278dd416ef2fc6ff3ac2586199fba5c1af
Reviewed-on: https://gerrit.instructure.com/46453
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

app/controllers/collaborations_controller.rb

@@ -55,12 +55,16 @@ class CollaborationsController < ApplicationController
   before_filter :require_collaborations_configured
   before_filter :reject_student_view_student
 
+  before_filter { |c| c.active_tab = "collaborations" }
+
   include Api::V1::Collaborator
 
   def index
     return unless authorized_action(@context, @current_user, :read) &&
       tab_enabled?(@context.class::TAB_COLLABORATIONS)
 
+    add_crumb(t('#crumbs.collaborations', "Collaborations"), polymorphic_path([@context, :collaborations]))
+
     @collaborations = @context.collaborations.active
     log_asset_access("collaborations:#{@context.asset_string}", "collaborations", "other")