Fix legislation proposals flag actions

We were treating legislation proposals as if they were proposals,
omitting the "legislation" namespace, and so we were flagging/unflagging
proposals when we wanted to flag/unflag a legislation proposal.

spec/system/legislation/proposals_spec.rb

@@ -8,6 +8,7 @@ describe "Legislation Proposals" do
 
   context "Concerns" do
     it_behaves_like "notifiable in-app", :legislation_proposal
+    it_behaves_like "flaggable", :legislation_proposal
   end
 
   scenario "Only one menu element has 'active' CSS selector" do