PaperTrail + Inheritance

[lazebny]

This PR resolves issues #1524, #2332, #2691
It is based on https://github.com/sferik/rails_admin/pull/1567/files.

PaperTrail stores a base_class for each subclass, so we can't fetch versions with searching by subclass. STI model allows us to fetch records with query condition which includes STI type, so model.model.all should return a valid AR relation for current STI type and we can fetch a versions by this relation.

When we have a deal with object versions think we can't do better than to use native PaperTrail method versions, so object.versions should work well.

These changes were checked on our production db.

Here is illustration how PaperTrail works:

class PaperTrailTest; end
class PaperTrailTestSubclass < ::PaperTrailTest; end
class PaperTrailTest::SubclassInNamespace < PaperTrailTest; end
versions = PaperTrail::Version.where(item_type: 'PaperTrailTest')
PaperTrailTest.create
versions.count # 1
PaperTrailTestSubclass.create
versions.count # 2
PaperTrailTest::SubclassInNamespace.create
versions.count # 3

[vshmyhlo]

👍 i have same issue, would be nice to have these changes in master

[mshibuya]

Excellent, thanks!