Direct link from Alertmanager template to the "create silence" modal

We would like to add a link to a pre-filled silence modal (including all Prometheus labels) to our Alertmanager templates, so that our users can create silences for individual alerts very easily.

Direct links to a pre-filled create silence modal were implemented by #1906.

However, from looking at #1906, it seems that Karma expects the m parameter (containing the field values for the modal) to be base64-encoded. It seems that Alertmanager has no built-in support for base64: https://prometheus.io/docs/alerting/latest/notifications/.

Could we make the base64 optional, and also accept a regular url-encoded string for the m parameter?