Handle jsonschema references

[chriskapp]

If a schema name changes or it gets deleted we have maybe the case that other schemas still reference to the changed schema. This does not break the application since the complete schema is in the cache but then it is no longer possible to save the schema. So we should forbid renaming/deleting a schema if it is referenced by another schema.

[chriskapp]

Since we now only logical delete a schema this should be no problem