Validate richtext image file size and type from Backend

The API / Backend should not allow abitrary big files to be uploaded. For images inserted inline into richtext fields a file restriction of 20 MB seems appropriate.

Due to the current implementation blobs are persisted before checking validity. These would be cleaned up by the daily cleanup job (as they stay unattached). To immediately free disk space we also try to remove blobs if the object they are to be attached to is invalid.

For #1177 (closed)