Signed-off-by: Rémy Coutable <remy@rymai.me>

- Merged `AkismetSubmittable` into `Spammable`
- Clean up `SpamCheckService`
- Added tests for `Spammable`
- Added submit (ham or spam) options to `AkismetHelper`

- 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.