Update queue pop button functionality

to allow popping people from the queue that are pinned by others.

The pop order is now: pinned by you, not pinned, pinned by others, which
mirrors the way it is displayed in the UI.

Since the queue pop button can now pop up to $requests.length requests,
this closes #98.

app/models/course_queue.rb

@@ -41,6 +41,8 @@ def pop!(user)
       first_pinned.resolve_by!(user)
     elsif first_non_pinned = self.outstanding_requests.where(resolver: nil).first
       first_non_pinned.resolve_by!(user)
+    elsif first_pinned_by_others = self.outstanding_requests.where.not(resolver: user).first
+      first_pinned_by_others.resolve_by!(user)
     end
   end
 

test/models/course_queue_test.rb

@@ -91,8 +91,8 @@ class CourseQueueTest < ActiveSupport::TestCase
 
     assert request == course_queue_entries(:unresolved_entry)
 
-    # pinned by matt shouldn't be resolvable by jim
-    assert_nil @queue.pop!(users(:jim))
+    # if the queue is empty we will allow entries pinned by others to be popped
+    assert @queue.pop!(users(:jim)) == pinned_by_matt
   end
 
   test "request validates duplicates in group mode" do