Remove 'file' source type when HORIZON_IMAGES_ALLOW_UPLOAD is False

settings.HORIZON_IMAGES_ALLOW_UPLOAD is meant to hide everything
related to direct file uploads in the CreateImage dialog, but at
the moment it's just removing the image_file field and leaving
"File" as an option in the Source type dropdown.

This change removes the 'file' source type when necessary, and
hides the source type field altogether if there is only one source.

Change-Id: Idefb225bccb26fc4cf04f78387e205cb2fbacfd9
Closes-bug: 1251101
Merge "Remove 'file' source type when HORIZON_IMAGES_ALLOW_UPLOAD is False"