Url configurations for project and dataset slugs (3)

I've refactored the project's url configurations to work with the urls we discussed.

To see how this works, start with the root project urlconf and trace out the imports:

Main urlconf gets loaded first (message_coding/message_coding/urls.py)...

1. Defines `user_dash` url (this is removed in #23)
2. Imports admin url conf
3. Imports urls from "base" app (message_coding/base/urls.py)
4. Imports urls from "project" app (message_coding/apps/project/urls.py)
    1. Defines `project_create` url
    2. Loads the `project_slug_urls` from the same file.
        1. Load the `task_urls` from the same file.
            1. Defines `task_create` url
            2. Defines `task` detail url
        2. Imports the dataset urlconf from the "dataset" app (message_coding/apps/dataset/urls.py)
            1. Defines `dataset_import` url
            2. Defines `dataset` detail url
        3. Define the `project` detail url