Partially addresses #47424.

One should really use a separate table instead of using polymorphic
associations.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11168 for
more information.

- New concern `AkismetSubmittable` to allow issues and other `Spammable` models to be submitted to Akismet.
- New model `UserAgentDetail` to store information needed for Akismet.
- Services needed for their creation and tests.