Add question type: phone number (#1462)

* Add Phonenumber-Field as to Questions

* Add setup requirements

* Add list of ask-during-checkin restricted question types and enforce it

* Fix requirements

* Fix crash using custom locales

* Re-format phone numbers when outputting to humans

* Initialize country code field with a guess for the customer's country

* Document TEL type in API docs

src/pretix/helpers/json.py

@@ -1,5 +1,6 @@
 from django.core.files import File
 from i18nfield.utils import I18nJSONEncoder
+from phonenumber_field.phonenumber import PhoneNumber
 
 from pretix.base.reldate import RelativeDateWrapper
 
@@ -10,6 +11,8 @@ class CustomJSONEncoder(I18nJSONEncoder):
             return obj.to_string()
         elif isinstance(obj, File):
             return obj.name
+        if isinstance(obj, PhoneNumber):
+            return str(obj)
         else:
             return super().default(obj)