Correctly distinquish between parent types to avoid ID clashes

When selecting content that has been reacted to, discussion and comment
IDs will currently clash. E.g. if comment #16 has been reacted to, the
acted model will also query discussion #16 if it exists.

This commit fixes the bug by taking into account the reaction parent
types. The data was already available but wasn't being used.
Merge pull request #8 from kasperisager/actedmodel-content-type-fix

Thanks for catching my oversight. :D