Remove old bundle models & views and add first-cut integration with bundle service

This adds a basic, unstyled list view of bundles available in a bundle service and a page to see the details of a bundle. The URLs are `\bundles` for the list and `\bundles\<uuid>` for the details.

A bundle service needs to be deployed separately. In your local settings `local.py`, provide the URL to the bundle service by adding the following block of code:
```
# Bundle service
BUNDLE_SERVICE_URL = "http://your.bundleservice.net"
BUNDLE_SERVICE_CODE_PATH = "..\\..\\..\\..\\bundles" # path relative to this file
if len(BUNDLE_SERVICE_CODE_PATH) > 0:
    sys.path.append(join(dirname(abspath(__file__)), BUNDLE_SERVICE_CODE_PATH))
    codalab.__path__ = extend_path(codalab.__path__, codalab.__name__)
```
The `CODE_PATH` variable indicates the directory where current code for the bundle service is located on the machine running the Django app.