Fixing problems with events for import/export

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19202

A couple of issues related to target being missing in exported `Events` (as being polymorphic and not have `ActiveRecord` relationships is a bit more tricky than normal models) plus as the export was in JSON, the import retrieves hashed fields as stringified hashes and not symbolized - so fixed that as well, which was the cause of https://gitlab.com/gitlab-org/gitlab-ce/issues/19202

Also fixed / refactored tests
:simpl
Import/Export Version has been bumped to 0.1.1 as theses changes to events won't work very well with old exports - forcing users to generate a new export in the new version.

See merge request !4987
(cherry picked from commit c368cb60)