Add dependency on 'requests'
Move check_submit js to login page. No need to have it in base.pt.
Factor out login session information

The idea is to have one key on the session that corresponds to our login and
carries properties that determine what you can do. An anonymous user, for
example, would not be able to upload to cnx.
Add anonymous login control to login page.
Use AnonymousSession when logging in anonymously.