profile: Coupon codes are now service-specific

Obviously this won't work out of the box; at the minimum, you're going to need to run this in your database file:

`alter table discount_code add column service VARCHAR(4) DEFAULT 'vps';`

On line 30 of `invoice.py`, you can edit and add services to your liking. Those are grabbed and used in the admin page where you can manage coupons, as well as the actual database enum (though that doesn't have much effect unless you're actually creating the table, and we can't manually add the `CHECK` at this point.)

Any location that uses `get_discount_code` in `invoice.py` needs to be edited and a second argument needs to be added, one of the elements in the list on `invoice.py/L30` - "vps" or "dedi".

All current discount codes will be changed to apply only to vps'.