Added DELETE handler to files endpoint for deleting files and
directories, along with new handling for creating directories, as
specified with the `X-NAS-Type` header - if this is set to "directory",
a new directory is created with the path of the request. Otherwise, an
attempt to parse the file from form data is done as before.
Also added buttons to interact with the new functionality in the default
frontend.
Icons! Finally, signifying type of link without relying on colours. Also
added some margin to help alleviate strain from contrasting colours.
Fixed background-color of "NAS" home link maintaining white background.