Add S3 direct upload form.

https://github.com/madewithbytes/us_ignite/issues/71

The ``forms_s3.S3UploadForm`` component generates
a form with all the hidden fields and signatures
required to upload a file directly to S3.

The form can be configured to limit the size
and the details of the file uploaded.

A ``get_s3_upload_form`` helper is provided to
only receive a key and a redirect field.
Improve admin section for the ``Applications``.

The improvements make easier to debug the application
in the admin.
Add support to upload ``Application`` files.

https://github.com/madewithbytes/us_ignite/issues/71

The files will be uploaded and associated to
an ``Application`` object.