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/helpers/application_helper.rb

@@ -122,7 +122,9 @@ module ApplicationHelper
   end
 
   def time_ago_in_words_tagged(time, compact: false)
-    if time.past?
+    if time.nil?
+      tag.em(tag.time("unknown"))
+    elsif time.past?
       if compact
         text = time_ago_in_words(time)
         text = text.gsub(/almost|about|over/, "").strip