Admin rake task to create a default master user

This adds an admin namespaced rake task intended for development that can be
invoked with:

```ruby
bundle exec rake admin:create_master_user
```

This task creates a default school, a default FSDO, and a default user with a
master role. A developer wanting to get a local instance of flight record
configured quickly with the bare essentials can use this rake task to create a
master user with the 'admin' username.

When this task is invoked, the user will be prompted to choose a password for
the new 'admin' user.