HTML attr encoding for UploadField config

Also fixed UploadField operation with canUpload=false,
which broke the JS because no config could be found,
since the <input> field wasn't present. We can't put the data-config
attribute on the field holder, since fields can be rendered
without their field holder through FormField->SmalLFieldHolder().

Fixes https://github.com/silverstripe/silverstripe-cms/issues/874
Merge pull request #2558 from chillu/pulls/uploadfield-874

HTML attr encoding for UploadField config