ip bans: add hit counter, deleted flag, new ban type.

* Make IP bans soft deletable.
* Add a hit counter to track how many times an IP ban has blocked someone.
* Add a last hit timestamp to track when the IP ban last blocked someone.
* Add a new type of IP ban, the signup ban. Signup bans restrict new
  signups from editing anything until they've verified their email
  address.

app/views/ip_addresses/_info.html.erb

@@ -26,7 +26,7 @@
     <tr>
       <th>IP Banned</th>
       <td>
-        <% if IpBan.is_banned?(ip_address.ip_addr.to_s) %>
+        <% if IpBan.ip_matches(ip_address.ip_addr.to_s).exists? %>
           yes (<%= link_to "info", ip_bans_path(search: { ip_addr: ip_address.to_s }) %>)
         <% else %>
           no