Allow to hide a product unless a specific quota is sold out (#1351)

* Allow to hide a product unless a specific quota is sold out

* Fix required property

* Add API property and copy between events

doc/user/events/structureguide.rst

@@ -45,8 +45,8 @@ In addition, you will need quotas. If you do not care how many of your tickets a
 
 If you want to limit the number of student tickets to 50 to ensure a certain minimum revenue, but do not want to limit the number of regular tickets artificially, we suggest you to create the same quota of 200 that is linked to both products, and then create a **second quota** of 50 that is only linked to the student ticket. This way, the system will reduce both quotas whenever a student ticket is sold and only the larger quota when a regular ticket is sold.
 
-Use case: Early-bird tiers
---------------------------
+Use case: Early-bird tiers based on dates
+-----------------------------------------
 
 Let's say you run a conference that has the following pricing scheme:
 
@@ -58,9 +58,53 @@ Of course, you could just set up one product and change its price at the given d
 
 Create three products (e.g. "super early bird", "early bird", "regular ticket") with the respective prices and one shared quota of your total event capacity. Then, set the **available from** and **available until** configuration fields of the products to automatically turn them on and off based on the current date.
 
-.. note::
+Use case: Early-bird tiers based on ticket numbers
+--------------------------------------------------
 
-   pretix currently can't do early-bird tiers based on **ticket number** instead of time. We're planning this feature for later in 2019. For now, you'll need to monitor that manually.
+Let's say you run a conference with 400 tickets that has the following pricing scheme:
+
+* First 100 tickets ("super early bird"): € 450
+* Next 100 tickets ("early bird"): € 550
+* Remaining tickets ("regular"): € 650
+
+First of all, create three products:
+
+* "Super early bird ticket"
+* "Early bird ticket"
+* "Regular ticket"
+
+Then, create three quotas:
+
+* "Super early bird" with a **size of 100** and the "Super early bird ticket" product selected. At "Advanced options",
+  select the box "Close this quota permanently once it is sold out".
+
+* "Early bird and lower" with a **size of 200** and both of the "Super early bird ticket" and "Early bird ticket"
+  products selected. At "Advanced options", select the box "Close this quota permanently once it is sold out".
+
+* "All participants" with a **size of 400**, all three products selected and **no additional options**.
+
+Next, modify the product "Regular ticket". In the section "Availability", you should look for the option "Only show
+after sellout of" and select your quota "Early bird and lower". Do the same for the "Early bird ticket" with the quota
+"Super early bird ticket".
+
+This will ensure the following things:
+
+* Each ticket level is only visible after the previous level is sold out.
+
+* As soon as one level is really sold out, it's not coming back, because the quota "closes", i.e. locks in place.
+
+* By creating a total quota of 400 with all tickets included, you can still make sure to sell the maximum number of
+  tickets, even if e.g. early-bird tickets are canceled.
+
+Optionally, if you want to hide the early bird prices once they are sold out, go to "Settings", then "Display" and
+select "Hide all products that are sold out". Of course, it might be a nice idea to keep showing the prices to remind
+people to buy earlier next time ;)
+
+Please note that there might be short time intervals where the prices switch back and forth: When the last early bird
+tickets are in someone's cart (but not yet sold!), the early bird tickets will show as "Reserved" and the regular
+tickets start showing up. However, if the customers holding the reservations do not complete their order,
+the early bird tickets will become available again. This is not avoidable if we want to prevent malicious users
+from blocking all the cheap tickets without an actual sale happening.
 
 Use case: Up-selling of ticket extras
 -------------------------------------