Forms and views for project and dataset slugs (2)

Part 2 of #24. This is follow-up to #31.

Here, I've updated a bunch of the views for projects, datasets, and tasks so that they work with project and dataset slugs.

I moved the DatasetImportView and DatasetDetails view into the "dataset" app, instead of the "project" app where they were before.

The previous CreateProjectView and DatasetImportView were using the default ModelForms, but now I've added slightly customized forms (in the forms.py files) so that we can override some of the text related to slugs.

I'll add some more detailed explanations inline.
In general we have to specify the slug_url_kwarg because we aren't using the default kwarg ('slug'). Similarly for places where we use id numbers (like on tasks), we have to specify pk_url_kwarg.