Migrations and seeds

I added migrations for various tables:
* aircraft, formerly `aircraft_types`
* airlines
* airports
* atc
* countries, formerly `isocodes`
* flights
* pilots
* positions
* ratings
* registrations
* routes
* servers
* updates

The structure is based on the sample file in the legacy data repo and I don't know how the structure will be changed for the new version. Overall I have changed the structure a bit, primarily the naming of columns. No columns have been removed.

Each of these tables also come with their associated model (singular name of table, except for routes, which is called `Airroute` due to Laravel's `Route` class).

Furthermore, I have added seeding files for `Aircraft`, `Airlines`, `Airports`, `Countries`, and `Registrations` as those were made available in the legacy data repo. These can be seeded into the database by running `artisan db:seed --class=[name]TableSeeder`.