Turn "managed file" element into "advanced_file" element where managing is optional

[jenlampton]

We have a lot of places in core where we need a file-upload widget that does not result in a file entity, or need a managed file. Examples include:

• upload your logo
• upload your favicon
• default value for image/file fields

Let's make a generic ajax file upload form element that does not result in a managed file. We can decide if thie managed file element should "extend" this one (an optional flag?) or if the two elements should remain separate.