posts: factor out post edit logic.

Factor out most of the tag edit logic from the Post class to a new
PostEdit class. The PostEdit class contains the logic for parsing tags
and metatags from the tag edit string, and for determining which tags
were added or removed by the edit.

Fixes various bugs caused by not calculating the set of added or removed
tags correctly, for example when tag category prefixes were used (e.g.
`copy:touhou`) or when the same tag was added and removed in the same
edit (e.g. `touhou -touhou`).

Fixes #5123: Tag categorization prefixes bypass deprecation check
Fixes #5126: Negating a deprecated tag will still cause the warning to show
Fixes #3477: Remove tag validator triggering on tag category changes
Fixes #4848: newpool: metatag doesn't parse correctly

test/functional/posts_controller_test.rb

@@ -90,7 +90,7 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         end
 
         should "render for an artist tag" do
-          create(:post, tag_string: "artist:bkub", rating: "s")
+          as(@user) { create(:post, tag_string: "artist:bkub", rating: "s") }
           get posts_path, params: { tags: "bkub" }
           assert_response :success
           assert_select "#show-excerpt-link", count: 1, text: "Artist"
@@ -131,7 +131,7 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         end
 
         should "render for a tag with a wiki page" do
-          create(:post, tag_string: "char:fumimi", rating: "s")
+          as(@user) { create(:post, tag_string: "char:fumimi", rating: "s") }
           get posts_path, params: { tags: "fumimi" }
           assert_response :success
           assert_select "#show-excerpt-link", count: 1, text: "Wiki"

test/functional/related_tags_controller_test.rb

@@ -3,7 +3,7 @@ require 'test_helper'
 class RelatedTagsControllerTest < ActionDispatch::IntegrationTest
   context "The related tags controller" do
     setup do
-      create(:post, tag_string: "copy:touhou")
+      as(create(:user)) { create(:post, tag_string: "copy:touhou") }
     end
 
     context "show action" do