Change "Upload" to "Media" and revamp

[mutantmonkey]

The upload thing in the admin section of the site has significant room for improvement. I think we should rename it to "Media" and support some basic file management features as well.

I'd like to support uploads to the local filesystem and anything that supports the S3 API at a minimum. These should be configurable and show up under the existing "Destination" selector.

It could also be useful to have an option to support subdirectories in each of these, but that's not critical; if subdirectories are needed and this doesn't get implemented, they should just be configured as separate destinations. We're not going to support creation/deletion of subdirectories at this time.

After a file is uploaded, the URL should be provided to the user and also stored in the database. Below the uploader, we can have a basic paginated file manager with a list of all the current uploads and an option to delete. Table columns should probably be Destination, Filename, Upload Date, and Actions. The filename entry should have a link to the URL.

[mutantmonkey]

We could also have a checkbox to generate a thumbnail to take care of #234.