Static URL double-slash

While using vagrant-devstack, I noticed that certain URLs that were hard-coded to use "/static/" weren't loading.  This included links in the LMS footer template as well as links in the courseware (e.g. the Demo course about page)

It turns out that the aws.py settings were setting `STATIC_URL = "/static//"` with two trailing slashes.  In production, these links will resolve correctly because they are served by nginx.  When using django staticfiles, however, any URL that uses `/static/` (with only one trailing slash) will result in a 404.

This PR addresses the issue in two ways:
1) Append the trailing slash to `STATIC_URL_BASE` only if it doesn't already end in a slash.
2) Stop hardcoding static URLs in the LMS footer.  As far as I know, this page shows up only in the sandbox.