Fix comment notifications on legislation proposals

The `commentable_url` method wasn't updated when we added legislation
proposals.

Back when we first created this method, we couldn't pass budget
investments or topics directly to `polymorphic_url` because they are
nested resources. That isn't the case since commit ff93f5a59, so now we
can simplify this method.

We're keeping the `commentable_url` method for now in order to keep
compatibility with custom changes that might use it, although this
method isn't consistent with the `commentable_path` method (which
receives a comment, and not a commentable), and so we might have to
revisit this code in the future.

app/helpers/mailer_helper.rb

@@ -1,10 +1,6 @@
 module MailerHelper
   def commentable_url(commentable)
-    return poll_url(commentable) if commentable.is_a?(Poll)
-    return debate_url(commentable) if commentable.is_a?(Debate)
-    return proposal_url(commentable) if commentable.is_a?(Proposal)
-    return community_topic_url(commentable.community_id, commentable) if commentable.is_a?(Topic)
-    return budget_investment_url(commentable.budget_id, commentable) if commentable.is_a?(Budget::Investment)
+    polymorphic_url(commentable)
   end
 
   def valuation_comments_url(commentable)

spec/mailers/mailer_spec.rb

@@ -22,5 +22,11 @@ describe Mailer do
 
       expect(email).to deliver_from "New organization <new@consul.dev>"
     end
+
+    it "sends emails for comments on legislation proposals" do
+      email = Mailer.comment(create(:legislation_proposal_comment))
+
+      expect(email.subject).to include("commented on your proposal")
+    end
   end
 end