Url configurations for project and dataset slugs (3)

I've refactored the project's url configurations to work with the urls we discussed. This is part 3 for #24, building on #32.

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
This line loads all the urls that go underneath project slugs. Note that the concrete urls (e.g. "project/create") should be defined above the dynamic urls like this one, so that they take precedence.