Update project and dataset templates with new urls (4)

This is the final pull for #24 and also deals with #28. It updates the templates so that they use the url helper to reverse-lookup all urls by name instead of having them hard-coded (#28). It also provides project_slug and dataset_slug parameters where needed to conform with new slug-based urls.

The templates for the project creation and dataset import forms have been updated to include a slug field. This means that the user has to manually type them in -- which they probably should, but we can also auto-generate them from the name field.