Remove relations to Category, Tag and Media

[alexander-schranz]

Currently the Excerpt has a relation on database level to category, tag or media. This relations should not exist as the category, tag and media are in another context. Specially in case of on-delete cascade this make problems for Tags. Example if a Tag is merged into another Tag the Tag should not be on-delete cascade, as the event TagMerged and TagRemoved is thrown correctly after it is done. So if we want to merge tags then together or do anything else based on this we should save categories, tags and media not as database relations.