Add some user experience improvements to the uploads page (again)

[rhystmills]

Re-run of https://github.com/guardian/media-atom-maker/pull/1155 - without disabling the buttons. That PR was reverted because it inadvertently stopped users being able to activate YouTube live feeds, which media-atom-maker apparently supports.

The logic around this is not clear so I'm just going to introduce the more simple UI tweaks without touching this area. My guess is that YouTube live feeds never come out of the processing phase.

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

• 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

• Adds a message explaining why it's naughty to use the Asset URL option, to hopefully discourage the behaviour that led to videos not ending up in Pluto.

Before	After

• Adds a 'not-allowed' cursor for disabled buttons.

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.

1. Deploy this branch to CODE.
2. 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.
3. Click the pencil icon in the top right, which shows a tolltip saying 'Edit Assets' on hover.
4. 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?
5. Go back to the YouTube furniture tab and add a privacy status, e.g. unlisted.

[rhystmills]

Something's gone horribly wrong with this branch, going to create a PR from a new one.