Allow SVG club logos

ImageField validates uploads through Pillow, which can't read SVGs, so
a club crest that's a vector logo was rejected outright. Switches to a
plain FileField with an extension allowlist (png/jpg/jpeg/gif/webp/svg)
instead, and restricts the file picker to image types via the widget's
accept attribute.

controlpanel/forms.py

@@ -21,6 +21,7 @@ class ClubForm(forms.ModelForm):
         widgets = {
             "primary_color": forms.TextInput(attrs={"placeholder": "#1e40af"}),
             "secondary_color": forms.TextInput(attrs={"placeholder": "#be185d"}),
+            "logo": forms.ClearableFileInput(attrs={"accept": "image/png,image/jpeg,image/gif,image/webp,image/svg+xml"}),
         }
 
     def __init__(self, *args, **kwargs):