search: apply aliases after parsing searches.

Make PostQueryBuilder apply aliases earlier, immediately after parsing
the search.

On the post index page there are multiple places where we need to apply
aliases:

* When running the search with PostQueryBuilder#build.
* When calculating the search count with PostQueryBuilder#fast_count.
* When calculating the related tags for the sidebar.
* When tracking missed searches and popular searches for Reportbooru.
* When looking up wiki excerpts.

Applying aliases after parsing ensures we only have to apply aliases
once for all of these things.

We also normalize the order of tags in searches and strip repeated tags.
This is so that we have consistent cache keys for fast_count.

* Fixes searches for aliased tags being counted as missed searches (fixes #4433).
* Fixes wiki excerpts not showing up when searching for aliased tags.

test/functional/posts_controller_test.rb

@@ -68,6 +68,17 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           assert_select "#show-excerpt-link", count: 0
         end
 
+        should "render for an aliased tag" do
+          create(:tag_alias, antecedent_name: "/lav", consequent_name: "looking_at_viewer")
+          as(@user) { create(:wiki_page, title: "looking_at_viewer") }
+          @post = create(:post, tag_string: "looking_at_viewer", rating: "s")
+
+          get posts_path, params: { tags: "/lav" }
+          assert_response :success
+          assert_select "#post_#{@post.id}", count: 1
+          assert_select "#excerpt .wiki-link[href='/wiki_pages/looking_at_viewer']", count: 1
+        end
+
         should "render for a wildcard tag search" do
           create(:post, tag_string: "1girl solo")
           get posts_path(tags: "*girl*")