Add setting to control the creation of NullRevision on upload

Add setting to control the creation of NullRevision on upload

When uploading multiple file revisions with fitting text revisions to
core the UploadRevisionImporter will create a unwanted NullRevision on
the file page.

This NullRevision originates from LocalFile:upload() and is added there
in recordUpload2 as part of the normal upload process. There it is meant
to leave a hint on the text revision history.

This whole area is in need of heavy refactoring. But since the issue is
blocking the current implementation of the FileImporter extension, a
parameter is added to control the creation of that NullRevision.

Bug: T193621
Change-Id: I57c947eb63a7627ab1eec850cdf5e076f5f62df5