Use human_attribute_name in admin tables

To be consistent with the previous commit we update the text that appears
in the administration tables for discussions and proposals. Now instead of
"Proposals" and "Discussions" will appear "Title" which makes more sense
and is more consistent.

app/views/admin/debates/index.html.erb

@@ -12,7 +12,7 @@
   <table>
     <thead>
       <th class="text-center"><%= t("admin.debates.index.id") %></th>
-      <th><%= t("admin.debates.index.title") %></th>
+      <th><%= Debate.human_attribute_name(:title) %></th>
       <th><%= t("admin.debates.index.author") %></th>
     </thead>
     <tbody>

app/views/admin/proposals/index.html.erb

@@ -13,7 +13,7 @@
     <thead>
       <tr>
         <th class="text-center"><%= t("admin.proposals.index.id") %></th>
-        <th><%= t("admin.proposals.index.title") %></th>
+        <th><%= Proposal.human_attribute_name(:title) %></th>
         <th><%= t("admin.proposals.index.author") %></th>
         <th><%= t("admin.proposals.index.milestones") %></th>
         <th><%= t("admin.proposals.index.selected") %></th>