Require access to Still via method

This commit adds a safeguard to accessing the still from a specific
component by requiring access via the component.GetStill method.

internal/wisski/ingredient/php/users/policy.go

@@ -19,6 +19,7 @@ func (up *UserPolicy) Fetch(flags ingredient.FetcherFlags, target *status.WissKI
 	}
 
 	// read the grants into the info struct
-	target.Grants, err = up.Malt.Policy.Instance(flags.Context, up.Slug)
+	liquid := ingredient.GetLiquid(up)
+	target.Grants, err = liquid.Policy.Instance(flags.Context, liquid.Slug)
 	return err
 }

internal/wisski/ingredient/php/users/users.go

@@ -71,7 +71,7 @@ func (u *Users) LoginWithOpt(ctx context.Context, server *phpx.Server, username
 	}
 
 	// and resolve the (possibly relative) reference
-	dest = u.URL().ResolveReference(dest)
+	dest = ingredient.GetLiquid(u).URL().ResolveReference(dest)
 	return
 }