Add a secondary brand colour for clubs

Mirrors primary_color: a club-picked hex highlight used for accents like
the avatar-initials badge, with a computed readable text colour so a
pale pick doesn't produce white-on-yellow text. The shared contrast math
moves into _content_color_for so both colours use the same rule.

controlpanel/forms.py

@@ -13,12 +13,15 @@ from .services.admins import find_member_by_email
 class ClubForm(forms.ModelForm):
     class Meta:
         model = Club
-        fields = ["name", "slug", "logo", "primary_color"]
+        fields = ["name", "slug", "logo", "primary_color", "secondary_color"]
         help_texts = {"slug": _("Drives the club's subdomain. Left blank, it is derived from the name.")}
         # Deliberately a text input, not <input type="color">: a colour picker cannot
         # express "no colour" -- it would submit #000000 for every club that never
         # touched it, and every club would silently get a black theme.
-        widgets = {"primary_color": forms.TextInput(attrs={"placeholder": "#1e40af"})}
+        widgets = {
+            "primary_color": forms.TextInput(attrs={"placeholder": "#1e40af"}),
+            "secondary_color": forms.TextInput(attrs={"placeholder": "#be185d"}),
+        }
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)