Provide incorrect zip file warning

Intuitively, some users zip the entire folder with their map build, not its' contents.

Currently there's no warning that such a zip file will not work.

Suggestions:

1. Handle one-level nesting of zipped content (when the unzipped results are a single directory, use contents of the directory)
2. On file upload, warn the user that the zipped content contains no .xml file that can be used as a root map, suggesting incorrectly zipped build