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.

test/unit/post_query_builder_test.rb

@@ -1099,6 +1099,14 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
       assert_tag_match([], "exif:DNE")
     end
 
+    should "return posts for the random:<N> metatag" do
+      post = create(:post)
+
+      assert_tag_match([], "random:0")
+      assert_tag_match([post], "random:1")
+      assert_tag_match([post], "random:1000")
+    end
+
     should "return posts ordered by a particular attribute" do
       posts = (1..2).map do |n|
         tags = ["tagme", "gentag1 gentag2 artist:arttag char:chartag copy:copytag"]
@@ -1210,6 +1218,12 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
       end
     end
 
+    should "return posts for order:random" do
+      post = create(:post)
+
+      assert_tag_match([post], "order:random")
+    end
+
     should "return posts for a filesize search" do
       post = create(:post, file_size: 1.megabyte)