Prevent creating two reservations for the same product

[ArcoMul]

Currently some users use this way to 'extend' their reservation. Which is pretty smart, but would be better to add a proper way to extend a reservation.

[ArcoMul]

Proposal: when creating trying to create a reservation for a product which the user already has an upcoming or ongoing reservation for, show an error message saying:

"You already have a reservation for this product. Reach out on {email-address} to extend your reservation."

When the reservation overview has been implemented, we can redirect people there to prolong their ongoing reservation.

[maxverlaufen]

Translation:
"Du hast diesen Gegenstand bereits reserviert. Wenn du deine Reservierung verlängern möchtest, schreibe eine Mail an {email-adress}."