Add a loading indicator to the Files & Uploads page.

I've updated the assets page to use @frrrances's new loading indicator. This stops the page from jumping when the content loads, and tells the user that something is happening when the page is slow.

I also fixed up lint problems that PyCharm found, which included reindenting assets.js so apologies for all the whitespace-only changes.

@cahrens @frrrances Please review.