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.
Datasets are now shown at /<project-slug>/datasets/<dataset-slug> so they need the ProjectViewMixin to correctly parse the project from the url.