Fixed back button functionality

Addresses #254

We now store the previous URL if coming from a topic page and use that for the back button link.

app/controllers/offers_controller.rb

@@ -12,5 +12,11 @@ def show
     end
 
     @offer = Offer.find(params[:id])
+
+    previous_url = URI(request.referer || '').path
+    if previous_url.include? "topic"
+      session[:previous_url] = previous_url
+    end
+    @back_url = session[:previous_url]
   end
 end

app/views/offers/show.html.erb

@@ -1,7 +1,7 @@
 <%= render partial: 'projects/subnav' %>
 
 <div class="max-w-3xl mx-auto px-4 my-8">
-  <p class="mb-6"><%= link_to '‹ Back', :back, class: 'underline' %></p>
+  <p class="mb-6"><%= link_to '‹ Back', @back_url, class: 'underline' %></p>
   <%= render partial: 'layouts/notices' %>
 
   <div class="card">