Allow plugins to declare fonts "pdf-only"

src/pretix/base/settings.py

@@ -86,9 +86,9 @@ def primary_font_kwargs():
     from pretix.presale.style import get_fonts
 
     choices = [('Open Sans', 'Open Sans')]
-    choices += [
-        (a, {"title": a, "data": v}) for a, v in get_fonts().items()
-    ]
+    choices += sorted([
+        (a, {"title": a, "data": v}) for a, v in get_fonts().items() if not v.get('pdf_only', False)
+    ], key=lambda a: a[0])
     return {
         'choices': choices,
     }

src/pretix/presale/style.py

@@ -215,7 +215,8 @@ Return a dictionaries of the following structure. Paths should be relative to st
         },
         "bolditalic": {
             ...
-        }
+        },
+        "pdf_only": False,   # if True, font is not usable on the web
     }
 }
 """