Worksheets with permissions and My worksheet view.

This PR goes hand-in-hand with https://github.com/codalab/codalab-cli/pull/29. Worksheet permissions are reflected on the web site.

The main Worksheet list shows worksheet accessible by the current user. For the anonymous user (no one logged in), only worksheets shared with the public group are listed. Under My CodaLab, My worksheets tab lists the worksheets owned by the current user.

The main changes are:
- BundleService proxy class now takes in a user (maybe None for anonymous) and translates the user into an OAuth token that can be passed to the bundle service.
- The worksheet list view is extended to include for each item: actual user info, access info (read-only vs read-write), title and brief description.
- My worksheet list view shares the same controller as the main worksheet list view but the partial templates have difference.
- Worksheet view is also updated to show: 1) user; 2) title from the title directive in the CLI.