Stop offering deletion Slack refuses

chat.delete only removes what the bot posted itself, so webhook messages
answer cant_delete_message no matter how the id is found. Runs without a
stored id now say so instead of leading into a dead end.

src/lib/admin-settings.ts

@@ -120,6 +120,10 @@ export async function performDeleteMessage(viewer: Viewer, formData: FormData):
 		return invalid('runNotSent')
 	}
 
+	if (!run.messageTs) {
+		return invalid('runNotDeletable')
+	}
+
 	const values = await readSettings(['slack.botToken', 'slack.channel'])
 	const token = values.get('slack.botToken')?.trim()
 	const channel = run.channel ?? values.get('slack.channel')?.trim()