Add Range support to media fields

[davidjb]

Creates a custom field widget which has a "stream" view associated with it. This view correctly responds to partial content requests for media (via Plone's existing blob iterator), meaning seeking is now possible. Previously, a browser would need to download the entire video before this would be possible (or in Chrome's case, refuse to see at all).

Adding support for content ranges also ensures that iOS can view videos (see documentation requiring byte-range support)

[davidjb]

@vangheem: I wanted to run this by you since the package is yours.

[davidjb]

Separately, I was thinking this response support would be useful for standard plone.namedfile fields, much as there is support for byte-ranges in standard content types in Plone. For now, this solves the immediate issue.

[vangheem]

Sweet. Thanks!