search: make order:random truly random; add random:N metatag.

Make the `order:random` metatag truly randomize the search. Add a
`random:N` metatag that returns up to N random posts, like what
`order:random` did before.

`order:random` now returns the entire search in random order. Before it
just returned a pageful of pseudorandom posts. This will be more
accurate for small searches, but slower for large searches. If
`order:random` times out, try `random:N` instead.

The `random:N` metatag returns up to N pseudorandom posts. This is
faster than `order:random` for large searches, but for small searches,
it may return less than N posts, and the randomness may be biased. Some
posts may be more likely than others to appear. N must be between 0 and
200.

Also, `/posts?tags=touhou&random=1` now redirects to `/posts?tags=touhou+random:N`.
Before the `random=1` param acted like a free `order:random` tag; now it
redirects to a `random:N` search, so it counts against your tag limit.

app/views/posts/partials/index/_posts.html.erb

@@ -36,7 +36,5 @@
     <% end %>
   <% end %>
 
-  <% unless post_set.is_random? %>
-    <%= numbered_paginator(post_set.posts) %>
-  <% end %>
+  <%= numbered_paginator(post_set.posts) %>
 </div>