Change type of `objid` to text in table `vcategory_to_object`

For the `LocalUsers` () backend it must be possible to add strings to the `objid` column of the `vcategory_to_object` table. Refs #9179

I'm okay with merging this after oC7 is released since #8951 will also be merged after that, but can you tell me whether this change is okay?