fix: accept the Discord webhook URLs Discord issues, and anchor the pattern

Three separate problems with the pattern this replaces:

1. `discordapp.com` was rejected. That host is still served and still handed
   out, and the unescaped dot did not rescue it - "discord" plus any single
   character plus "com" does not match "discordapp.com". The workaround
   reporters found was to delete "app" from their own webhook URL.

2. The pattern was unanchored, and both ends check it with `RegExp.test`,
   which matches anywhere in the string. Any URL at all that merely contained
   a webhook-shaped substring was accepted - and the stored value is where
   the speedtest results are then posted.

3. The leading `.*` accepted any host ending in the pattern, so
   `notdiscord.com` passed.

The replacement also allows the `/api/v10/webhooks/...` form and Discord's
own `?thread_id=` parameter, both of which the old pattern rejected.

This fixes the half of #1297 about the URL not saving. Several reporters
also say the webhook saves and then never delivers; that is not addressed
here.

Refs #1297

Author: i7Gamer

server/integrations/discord.js

@@ -26,7 +26,8 @@ export default (registerEvent) => {
     return {
         icon: "fa-brands fa-discord",
         fields: [
-            {name: "url", type: "text", required: true, regex: /https:\/\/.*discord.com\/api\/webhooks\/\d+\/.+/},
+            {name: "url", type: "text", required: true,
+                regex: /^https:\/\/(?:ptb\.|canary\.)?discord(?:app)?\.com\/api(?:\/v\d+)?\/webhooks\/\d+\/[\w-]+(?:\?\S*)?$/},
             {name: "display_name", type: "text", required: false},
             {name: "send_finished", type: "boolean", required: false},
             {name: "finished_message", type: "textarea", required: false},