Allow ticket QR code colour to be configured (#4726)

* Allow ticket QR code colour to be configured

This commit introduces a feature enabling users to customise the QR
code colour in the ticket editor.

* Remove redundant argument from `Dict.get` call

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

---------

Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

src/pretix/plugins/ticketoutputpdf/models.py

@@ -37,7 +37,13 @@ DEFAULT_TICKET_LAYOUT = '''[
         "content": "secret",
         "text": "",
         "text_i18n": {},
-        "nowhitespace": false
+        "nowhitespace": false,
+        "color": [
+            0,
+            0,
+            0,
+            1
+        ]
     },
     {
         "type": "poweredby",