Refuse a bot token without a channel

Saving a token and leaving the channel empty produced a state that could
neither send nor delete, and only said so at the next attempt.

src/lib/admin-settings.ts

@@ -36,6 +36,13 @@ export async function performSaveSlack(viewer: Viewer, formData: FormData): Prom
 		return invalid('tokenInvalid', undefined, { botToken: 'tokenInvalid' })
 	}
 
+	const stored = await readSettings(['slack.botToken'])
+	const withToken = keepToken ? Boolean(stored.get('slack.botToken')) : token !== ''
+
+	if (withToken && channel === '') {
+		return invalid('channelRequired', undefined, { channel: 'channelRequired' })
+	}
+
 	if (!keep) {
 		if (webhook === '') {
 			await clearSetting('slack.webhook')