Auto-create _matrix webhook in channel.

src/bot.ts

@@ -194,6 +194,10 @@ export class DiscordBot {
     if (botUser) {
       const webhooks = await chan.fetchWebhooks();
       hook = webhooks.filterArray((h) => h.name === "_matrix").pop();
+      // Create a new webhook if none already exists
+      if (!hook) {
+        hook = await chan.createWebhook("_matrix", "", "Matrix Bridge: Allow rich user messages");
+      }
     }
     try {
       if (!botUser) {