Allow embedding by list of trusted referers when GLOBAL_LOGIN is enabled.

[yatesdr]

1). Add a settings key to optionally allow embedding videos from a list of trusted referers when GLOBAL_LOGIN_REQUIRED is true. This is useful if you wish to use MediaCMS as a back-end content manager, with the embedded files being used on another site.

2). If above option is set, modify the auth-bypass regex list in settings.py to permit loading from all origins in the embed view without auth, to paths in r'^/embed.*' and r'^/media/.*'

3). Modify embed view to validate embed requests against the list of allowed referers - check request.META['HTTP_REFERER'], parse the domain, and compare to the list. This required including urlparse to perform the domain parse in a reliable and standard way in this view.

4). Create a "fake player" to render (templates/cms/embed-403.html) with a mock 403 error, that fits to parent element and is generally styled like a generic player element. This is required since embedding in rich content editors often does not properly set referrer on otherwise trusted sites, and giving a 500 API error or redirecting to the MediaCMS main login page is not desirable in this use case.

Example "Bad referrer" render in the stock embed iframe:

5). Doc updates to reflect this new option and its usage.

[yatesdr]

Ping on the draft PR - Just clicked "ready for review".