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/controllers/posts_controller.rb

@@ -8,9 +8,13 @@ class PostsController < ApplicationController
       respond_with(@post) do |format|
         format.html { redirect_to(@post) }
       end
+    elsif params[:random].to_s.truthy?
+      post_set = PostSets::Post.new(params[:tags], params[:page], params[:limit], format: request.format.symbol, view: params[:view])
+      query = "#{post_set.normalized_query.to_s} random:#{post_set.per_page}".strip
+      redirect_to posts_path(tags: query, page: params[:page], limit: params[:limit], format: request.format.symbol, view: params[:view])
     else
       tag_query = params[:tags] || params.dig(:post, :tags)
-      @post_set = PostSets::Post.new(tag_query, params[:page], params[:limit], random: params[:random], format: params[:format], view: params[:view])
+      @post_set = PostSets::Post.new(tag_query, params[:page], params[:limit], format: request.format.symbol, view: params[:view])
       @posts = authorize @post_set.posts, policy_class: PostPolicy
       @post_set.log!
       respond_with(@posts) do |format|