Add airport table and data for worldwide airports

[flymia]

This patch fixes #7 and includes the following:

• Creates a new airport table
• Fills the data using seeders, when running the db:seed command - This works by supplying a .sql-file which gets loaded into the table.
• Adds an airport model with datatypes
• Changes the references of airports to the respective foreign keys

[flymia]

@Konrad00 This is the PR for #7.

I've taken the data from ourairports.com and modified it the following:

• Remove heliports
• Remove every airport, which does not include an icao_code
• Remove every closed airport, since we don't want to include historical data
• Imported it to MySQL and modified the table names, created a migration for it

[flymia]

I will go ahead and merge this, since this does not really break anything, that we have right now.