Create admin dashboard page for application-wide settings

[GabrielMajeri]

We should create a page in the admin interface (for example, at the /admin/settings path) to allow for the viewing and changing of application-wide settings.

For now, the only setting we'll need to implement is didactic_career_premium_euplatesc_account_id, which should display a select box allowing the admin to choose which EuPlătesc account is to be used as a destination for payment with a didactic career premium card.