Remove default ntfy server setting

Setting this to "https://ntfy.sh" has security implications: If the user
forgets to set his server, but uses the new short form for topics, the
notification will be sent to "ntfy.sh" and could expose information.

config.scfg

@@ -65,7 +65,7 @@ resolved {
 
 ntfy {
     # URL of the ntfy server.
-    # Default: "https://ntfy.sh"
+    # Default: ""
     server https://ntfy.sh
     # Name of the ntfy topic. For backwards compatibility you can specify the full URL of the
     # topic (e.g. https://ntfy.sh/alertmanager-alerts) and the server will be parsed from it.

config/config.go

@@ -430,8 +430,6 @@ func Read(path string) (*Config, error) {
 	config.LogFormat = "text"
 	config.AlertMode = Multi
 
-	config.Ntfy.Server = "https://ntfy.sh"
-
 	config.Cache.Type = "disabled"
 	config.Cache.Duration = time.Hour * 24
 	// memory

config/config_test.go

@@ -47,6 +47,7 @@ resolved {
 }
 
 ntfy {
+    server https://ntfy.sh
     topic https://ntfy.sh/alertmanager-alerts
     certificate-fingerprint 13:6D:2B:88:9C:57:36:D0:81:B4:B2:9C:79:09:27:62:92:CF:B8:6A:6B:D3:AD:46:35:CB:70:17:EB:99:6E:28:08:2A:B8:C6:79:4B:F6:2E:81:79:41:98:1D:53:C8:07:B3:5C:24:5F:B1:8E:B6:FB:66:B5:DD:B4:D0:5C:29:91
     user user

main.go

@@ -637,6 +637,10 @@ func (br *bridge) topicURL(topic string) (string, error) {
 		return topic, nil
 	}
 
+	if br.cfg.Ntfy.Server == "" {
+		return "", errors.New("cannot set topic: no ntfy server set")
+	}
+
 	s, err := url.JoinPath(br.cfg.Ntfy.Server, topic)
 	if err != nil {
 		return "", err