Generating YAML used by main Software Carpentry website.

1.  Added `slug` field to `Person` (will usually be lowercase lastname.firstname) for backward compatibility.
    Only instructors need to have slugs - these are used in filenames (e.g., lastname.firstname.json is the JSON for the instructors' badge).

2.  Updated `migrater.py` and `db.sql` to reflect this change.

3.  Added links from `index.html` to displays of exported data.

4.  Added page to display exported data.

5.  Added view and helper functions to generate exported data.

6.  Modified `urls.py` to glue them all together.

To do:

1.  Pretty-print the YAML (it ought to be indented nicely).

2.  Aggregate airports so that we can count the number of instructors at each.

3.  Only select airports currently matching instructors.