web: replace certbot with acme.sh

The former seems to be in a pretty bad state for usage with Debian based
containers:

- The Debian provided package is too old
- certbot-auto no longer works on Debian
- The recommended way of using snap is not Docker friendly

Thus, we are migrating to acme.sh, which has the advantage of also
making the web container slimmer.

env.example

@@ -71,6 +71,9 @@ TZ=UTC
 # E-Mail for receiving important account notifications (mandatory)
 #LETSENCRYPT_EMAIL=alice@atlanta.net
 
+# Use the staging server (for avoiding rate limits while testing)
+#LETSENCRYPT_USE_STAGING=1
+
 
 #
 # Etherpad integration (for document sharing)