tags: track tag histories.

Track the history of the tag `category` and `is_deprecated` fields in
the `tag_versions` table.

Adds generic Versionable and VersionFor concerns that encapsulate most
of the history tracking logic. These concerns are designed to make it
easy to add history to any model.

There are a couple notable differences between tag versions and other versions:

* There is no 1 hour edit merge window. All changes to the `category`
  and `is_deprecated` fields produce a new version in the tag history.

* New versions aren't created when a tag is created. Versions are only
  created when a tag is edited for the first time. The tag's initial
  version isn't created until *after* the tag is edited for the first time.

For example, if you change the category of a tag that was last updated
10 years ago, that will create an initial version of the tag backdated
to 10 years ago, plus a new version for your edit.

This is for a few reasons:

* So that we don't have to create new tag versions every time a new tag
  is created. This would be wasteful because most tags never have their
  category or deprecation status change.
* So that if you make a typo tag, your name isn't recorded in the tag's
  history forever.
* So that we can create new tags in various places without having to know
  who created the tag (which may be unknown if the current user isn't set).
* Because we don't know the full history of most tags, so we have to
  deal with incomplete histories anyway.

This has a few important consequences:

* Most tags won't have any tag versions. They only gain tag versions if
  they're edited.
* You can't track /tag_versions to see newly created tags. It only
  shows changes to already existing tags.
* Tag version IDs won't be in strict chronological order. Higher IDs may
  have created_at timestamps before lower IDs. For example, if you
  change the category of a tag that is 10 years old, that will create an
  initial version with a high ID, but with a created_at timestamp dated
  to 10 years ago.

Fixes #4402: Track tag category changes

test/unit/post_test.rb

@@ -345,6 +345,22 @@ class PostTest < ActiveSupport::TestCase
         @post = FactoryBot.create(:post)
       end
 
+      context "with a new tag" do
+        should "create the new tag" do
+          tag1 = create(:tag, name: "foo", post_count: 100, category: Tag.categories.character)
+          create(:post, tag_string: "foo bar")
+          tag2 = Tag.find_by_name("bar")
+
+          assert_equal(101, tag1.reload.post_count)
+          assert_equal(Tag.categories.character, tag1.category)
+          assert_equal(0, tag1.versions.count)
+
+          assert_equal(1, tag2.post_count)
+          assert_equal(Tag.categories.general, tag2.category)
+          assert_equal(0, tag2.versions.count)
+        end
+      end
+
       context "with a banned artist" do
         setup do
           CurrentUser.scoped(FactoryBot.create(:admin_user)) do
@@ -490,7 +506,7 @@ class PostTest < ActiveSupport::TestCase
         should "not remove the tag if the tag was already in the post" do
           bad_tag = create(:tag, name: "bad_tag")
           old_post = create(:post, tag_string: "bad_tag")
-          bad_tag.update!(is_deprecated: true)
+          bad_tag.update!(is_deprecated: true, updater: create(:user))
           old_post.update!(tag_string: "asd bad_tag")
 
           assert_equal("asd bad_tag", old_post.reload.tag_string)
@@ -525,9 +541,30 @@ class PostTest < ActiveSupport::TestCase
       context "tagged with a metatag" do
         context "for a tag category prefix" do
           should "set the category of a new tag" do
-            create(:post, tag_string: "char:hoge")
+            create(:post, tag_string: "char:chen")
+            tag = Tag.find_by_name("chen")
 
-            assert_equal(Tag.categories.character, Tag.find_by_name("hoge").category)
+            assert_equal(Tag.categories.character, tag.category)
+            assert_equal(0, tag.versions.count)
+          end
+
+          should "change the category of an existing tag" do
+            user = create(:user)
+            tag = create(:tag, name: "hoge", post_count: 1)
+            post = as(user) { create(:post, tag_string: "char:hoge") }
+
+            assert_equal(Tag.categories.character, tag.reload.category)
+
+            assert_equal(2, tag.versions.count)
+            assert_equal(1, tag.first_version.version)
+            assert_nil(tag.first_version.updater)
+            assert_nil(tag.first_version.previous_version)
+            assert_equal(Tag.categories.general, tag.first_version.category)
+
+            assert_equal(2, tag.last_version.version)
+            assert_equal(user, tag.last_version.updater)
+            assert_equal(tag.first_version, tag.last_version.previous_version)
+            assert_equal(Tag.categories.character, tag.last_version.category)
           end
 
           should "change the category for an aliased tag" do