Unify code in debates/legislation vote links

We were using the same code to render links to agree and disagree, so we
can extract a new component for this code.

We're also adding component tests to make it easier to test whether
we're breaking anything while refactoring, although the code is probably
already covered by system tests.

Since the votes mixin was only used in one place, we're removing it and
moving most of its code to a new CSS file for the shared component.

app/components/legislation/proposals/votes_component.rb

@@ -1,6 +1,6 @@
 class Legislation::Proposals::VotesComponent < ApplicationComponent
   attr_reader :proposal
-  delegate :css_classes_for_vote, :current_user, :link_to_verify_account, :votes_percentage, to: :helpers
+  delegate :current_user, :link_to_verify_account, to: :helpers
 
   def initialize(proposal)
     @proposal = proposal
@@ -8,10 +8,6 @@ class Legislation::Proposals::VotesComponent < ApplicationComponent
 
   private
 
-    def voted_classes
-      @voted_classes ||= css_classes_for_vote(proposal)
-    end
-
     def can_vote?
       proposal.votable_by?(current_user)
     end