Refresh extension listing on upload; show publish date in listing

Also addresses issues from my previous pull and includes a few other code cleanups--might want to review the commits separately.

Note that I'm using "old-style" AJAX to refresh the extension listing (just having the server regenerate the HTML and send it back, rather than doing client-side templating), since it didn't seem worth having the same logic on both the client and server for such a simple use case.

@dangoor