In the field value updater, add support for updating document content

The field value updater allows today to change the value of a property of an object of a document on all the wikis of the farm.
We should also add support for changing the content of a document on all documents of the farm.

Also, other document metadata should be modifiable as well, such as document title, document parent, etc, but separate issues should be created for these, this issue is about the content.