Widgets: Enable oEmbed support for Video widget

[westonruter]

With #34115 now resolved, we can now support oEmbeds in the video widget instead of limiting the widget to just playing what is supported by MediaElement.js. See previously #40935 where the video widget was explicitly limited to support YouTube and Vimeo since these were the only hosted video providers that MediaElement.js supported directly without needing to resort to oEmbed.

See #39994 for the introduction of the video widget.

Note also that oEmbeds are now supported in the rich Text widget as well, so there may need to be some changes done for that widget in particular to apply the same inject_video_max_width_style logic that the Video widget is applying now to the Video shortcode output.

Work in progress to get another contributor started

@timmydcrawford Is this something you'd be interested in picking back up to round out your work on the Video widget?

@westonruter what is the timeline for the task / when is it needed by? Sorry kind of unplugged from the 4.9 calendar.

@timmydcrawford beta1 is due Wednesday (in 2 days), so that would be the timeline.

@westonruter @melchoyce What do you think of 42039.2.diff​?

In 41759:

Widgets: Allow oEmbeds in video widget.

Opens up video embeds to all supported video oEmbed providers.

Props westonruter.
See #42039.

In 41765:

Widgets: Continue to use shortcode for some embeds

Renders YouTube and Vimeo embeds through WordPress' video shortcode.

See #42039.

We can follow-up with fixing the rendering of the preview in a defect ticket.

In 41827:

Widgets: Update video widget embed failure notice to indicate streams other than YouTube and Vimeo are allowed.

Amends [41759].
See #42039.

In 41933:

Widgets: Harden logic for checking for valid media URLs.

• Update deprecated isHostedVideo method to always return true since all oEmbeds are now supported.
• Disable scanImage for non-image media widgets.
• Ensure embed URL field element is matched from current media frame instance due to bug where media frames are not destroyed upon closing.
• Update error message for invalid video URLs to ask user to check the URL for validity.

Props gk.loveweb, octalmage, westonruter.
See #42039, #40935.
Fixes #41052.