Make Media Youtube's WYSIWYG insert autosubmit code more specific, non-destructive

Problem/Motivation

Right now Media Youtube sets the autosubmit setting for WYSIWYG inserts to true if the file is a video and false otherwise. Two problems:

1. It hijacks the autosubmit setting for all video files, preventing any custom settings for vimeo/local videos/etc.
2. It overwrites any autosubmit value set before it, i.e. if you have a module that sets image file autosubmit to true and Media Youtube's hooks are called afterward it will check if the file is a video and if not set autosubmit to false. Bummer.

Proposed resolution

Check to see if mimetype is media/youtube, and if so set autosubmit to true. More specific and only fires on youtube files.

Related issue for Media Vimeo at #1572790: Make Media Vimeo's WYSIWYG insert autosubmit code more specific, non-destructive.

Patch to follow.