users: reword ban notice messages.

* Show the ban length instead of the ban expiration date in ban notices.
* Fix the ban notice to not say "Your account has been temporarily
  banned" when it's a permanent ban.

app/views/bans/index.html.erb

@@ -15,7 +15,7 @@
         </div>
       <% end %>
       <% t.column "Duration" do |ban| %>
-        <%= humanized_duration(ban.created_at, ban.expires_at) %>
+        <%= humanized_duration(ban.duration) %>
       <% end %>
       <% t.column "Banner" do |ban| %>
         <%= link_to_user ban.banner %>