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
54a45a3021