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

app/controllers/tags_controller.rb

@@ -26,7 +26,7 @@ class TagsController < ApplicationController
 
   def update
     @tag = authorize Tag.find(params[:id])
-    @tag.update(permitted_attributes(@tag))
+    @tag.update(updater: CurrentUser.user, **permitted_attributes(@tag))
     respond_with(@tag)
   end
 end