Use absolute template paths

Bug: When trying to evoke `manage.py` from outside the repo root, the workbench fails to find the templates.
This is important in several cases, such as selenium integration tests, that are usually ran from outside the repository.

Steps to reporduce (assumed you are in the repo root):
```bash
cd ../; xblock-sdk/manage.py syncdb <<<"no"; xblock-sdk/manage.py runserver;
```
You are right, this was an overshight on my part. (Although, strangely pylint did not complain about it.)