Add monitor name context to Slack fallback text.

The text block of a slack notification payload is used for mobile
devices and plain text previews. This change allows slack users to see
the name of the failing service without having to open up Slack to read
the entire message.

server/notification-providers/slack.js

@@ -39,8 +39,9 @@ class Slack extends NotificationProvider {
             }
 
             const time = heartbeatJSON["time"];
+            const textMsg = "Uptime Kuma Alert";
             let data = {
-                "text": "Uptime Kuma Alert",
+                "text": monitorJSON ? textMsg + `: ${monitorJSON.name}` : textMsg,
                 "channel": notification.slackchannel,
                 "username": notification.slackusername,
                 "icon_emoji": notification.slackiconemo,