API File->canEdit() returns TRUE by default (not checking CMS perms)

This is a measure to support form fields and controllers
interacting with files in different contexts,
for example an UploadField used in a ModelAdmin,
or a website frontend. The check for 'CMS_ACCESS_AssetAdmin'
was too restricting. This wasn't a problem in 2.x simply because
the old FileField/Upload classes didn't respect File->can*()
permissions.
Merge pull request #1 from halkyon/merging_file_canedit_check_from_31

Merging File canEdit check from 3.1