Show (some) discounts in product list

Discounts are currently only shown in the cart, after products have been
selected. This was not a big problem since we invented the discount
machinery specifically for group discounts, which can only be applied in
the cart. However it *is* possible to define a Discount that applies to
a group of one. Then, one would expect that the discounted price would
be shown already on the front page of the shop.

Back then, I decided against this for complexity and performance
concerns, but after looking into it, it's not actually that bad.

On the user side, I think this is a game changer for the discount
feature. Now, I could use discount rules also to implement early-bird
pricing without duplicating producs/variations, or I can do a christmas
discount on all subevents in december without modifying all individual
subevents.

src/pretix/base/models/items.py

@@ -510,7 +510,9 @@ class Item(LoggedModel):
         verbose_name=_("Free price input"),
         help_text=_("If this option is active, your users can choose the price themselves. The price configured above "
                     "is then interpreted as the minimum price a user has to enter. You could use this e.g. to collect "
-                    "additional donations for your event.")
+                    "additional donations for your event. We recommend against combining this feature with automatic "
+                    "discounts since discounts are applied as the last step of price computation, which means that the "
+                    "price entered by the customer will be modified again.")
     )
     free_price_suggestion = models.DecimalField(
         verbose_name=_("Suggested price"),