search: refactor fast_count to return nil on timeout.

* Refactor fast_count to return nil instead of 1,000,000 if the exact count times out.
* Remove the estimate_post_counts and blank_tag_search_fast_count global config options.
* Replace the hardcoded post count estimates inside fast_count with a
  method that parses Postgres's estimated row count from EXPLAIN.

* /counts/posts.json:
** Remove the `raise_on_timeout` parameter.
** Add an `estimate_count=<true|false>` parameter.
** Return null instead of 1,000,000 if the exact count times out.

app/views/counts/posts.html.erb

@@ -4,7 +4,7 @@
 <div id="c-counts">
   <div id="a-posts">
     Post count for <%= link_to params[:tags].present? ? params[:tags] : "/posts", posts_path(:tags => params[:tags]) %>:
-    <% if @count == (Danbooru.config.blank_tag_search_fast_count || 1_000_000) %>
+    <% if @count.nil? %>
       timed out
     <% else %>
       <%= @count %>