NEW Restrict upload abilities in UploadField

Conflicts:
	css/UploadField.css
	templates/UploadField.ss
NEW Global default config for UploadField
Show label-less input#file field in UploadField with canUpload=false

The field carries the configuration, and some non-upload functionality
like "attach files" still relies on the fileupload jQuery plugin
being initialized.

Conflicts:
	templates/UploadField.ss
FIX: Allow images attached to UploadFields to be unlinked without File::canEdit() or File::canDelete() permission.

Although editing meta-data or deleting permanently would require File editing/deleting permissions, merely linking to a record does not.  This change is important for allowing front-end use of UploadField; or, more importantly, use of UploadFile by people without CMS rights.
NEW: Added canAttachExisting config option for UploadField.

This is the companion setting to canUpload, letting you control whether existing files from the asset store can be referenced.  It's particularly useful when using UploadField on the front-end.
NEW: Added replaceExistingFile setting for UploadField.

Sometimes has-one UploadFields can get confused about whether or not there is an existing file that needs deleting.  This setting lets you make a more robust has-one UploadField, where any existing file will be replaced.  It more closely mimics simple single-file upload fields.
FIX: Fixed margins so that margin is displayed between preview images and their title.

Previously the margin was erroneously only shown for a drop zone.  It's possible that this bug only affects single-image uploads.
Fixed UploadField regression from 4da1af9c3