Raise tag limit for objects

With the increased use of tags esp. for semantic distinction of posts
the current limit turns out to be not sufficient when semantic tags are
mixed with user input. There might be an opportunity to split those
apart to avoid unnecessary side-effects.

service/object/object.go

@@ -175,7 +175,7 @@ func (o *Object) Validate() error {
 		)
 	}
 
-	if len(o.Tags) > 5 {
+	if len(o.Tags) > 25 {
 		return wrapError(ErrInvalidObject, "too many tags")
 	}