Add some user experience improvements to the uploads page

This PR fixes some bits of awkward functionality on the uploads page, specifically:

The video previews on the uploads are bigger, accommodate more text by default and the description bar expands to accommodate longer text. Before, they took up an oddly small amount of screen space and consequently the information about the progress through the upload process was usually unreadable.

Before	After

Disables the 'Activate' and 'Delete' buttons until the video processing is done. Clicking on these before the video is processed wasn't functional and gave a misleading error ('Could not revert asset'). I've added a tooltip explaining why they're disabled on hover.

Before	After

Adds a message explaining why the 'Upload to Youtube' 'Choose file' button is disabled when it is - previously it wasn't clear (though there was a suitable error message in the code that wasn't ever rendered). Also removes the hover styling when it's disabled.

Before	After

How to test

This PR will be simpler to test on CODE because getting the end-to-end YouTube upload working locally will be complicated.

Deploy this branch to CODE.
Fill out the 'YouTube Furniture' without setting the Privacy status. This will let you check the validation is showing properly in the 'Edit Assets' page.
Click the pencil icon in the top right, which shows a tolltip saying 'Edit Assets' on hover.
Is there an explanation of why you can't upload a video? On hover, does the 'choose file' button under 'Upload to YouTube' remain the same colour?
Go back to the YouTube furniture tab and add a privacy status, e.g. unlisted.
Try uploading a .mp4 video. Are the Activate and Delete buttons disabled with a tooltip until the processing is done? Can you read the various messages shown while the processing takes place?