Added URL to assets

Field asset_url was added to Asset model.

This fields contains the URL the user should be redirected to when a
reservation is active.

An "Activate" button was added to active reservations in reservation view, so
users can access the external asset from this view.
ReservationResource modified

Fields remaining_time and active_in were added and API key authentication was
enabled to allow external asset to check if a reservation is valid.