Add uncategorised cookie category

Replace the `required` boolean column on cookie_categories with a `kind`
enum (NORMAL, NECESSARY, UNCATEGORISED). The Necessary category remains
undeletable and always-on for consent; the new Uncategorised category is
also undeletable but users can opt out of it.

When a category is deleted, its cookies are merged into the Uncategorised
category (lazy-created for legacy banners that don't have one yet).

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/coredata/cookie_banner_version.go

@@ -37,10 +37,10 @@ type (
 	}
 
 	CookieBannerVersionSnapshotCategory struct {
-		Name        string      `json:"name"`
-		Description string      `json:"description"`
-		Required    bool        `json:"required"`
-		Cookies     CookieItems `json:"cookies"`
+		Name        string             `json:"name"`
+		Description string             `json:"description"`
+		Kind        CookieCategoryKind `json:"kind"`
+		Cookies     CookieItems        `json:"cookies"`
 	}
 
 	CookieBannerVersion struct {