🎨 Allow any Slack-compatible webhook URLs in Slack app

closes https://github.com/TryGhost/Ghost/issues/9246, closes https://github.com/TryGhost/Ghost/issues/7856
- allow any valid URL in the slack webhook input
- update slack app description to mention Slack-compatible services including Discord and Mattermost

ghost/admin/app/templates/settings/apps/slack.hbs

@@ -34,7 +34,7 @@
             <div class="gh-setting">
                 <div class="gh-setting-content">
                     <div class="gh-setting-title">Webhook URL</div>
-                    <div class="gh-setting-desc">Automatically send newly published posts to a channel in Slack</div>
+                    <div class="gh-setting-desc">Automatically send newly published posts to a channel in Slack or any Slack-compatible service like Discord or Mattermost.</div>
                     <div class="gh-setting-content-extended">
                         {{#gh-form-group errors=model.errors hasValidated=model.hasValidated property="url"}}
                             {{gh-input model.url name="slack[url]" update=(action "updateURL") onenter=(action "save") focusOut=(action "triggerDirtyState") placeholder="https://hooks.slack.com/services/..." data-test-slack-url-input=true}}

ghost/admin/app/validators/slack-integration.js

@@ -1,4 +1,5 @@
 import BaseValidator from './base';
+import {isBlank} from '@ember/utils';
 
 export default BaseValidator.create({
     properties: ['url'],
@@ -7,9 +8,8 @@ export default BaseValidator.create({
         let url = model.get('url');
         let hasValidated = model.get('hasValidated');
 
-        let urlRegex = new RegExp(/(^https:\/\/hooks\.slack\.com\/services\/)(\S+)/);
-
-        if (!validator.empty(url) && !url.match(urlRegex)) {
+        // eslint-disable-next-line camelcase
+        if (!isBlank(url) && !validator.isURL(url, {require_protocol: true})) {
             model.get('errors').add(
                 'url',
                 'The URL must be in a format like https://hooks.slack.com/services/<your personal key>'

ghost/admin/tests/unit/validators/slack-integration-test.js

@@ -47,7 +47,8 @@ describe('Unit: Validator: slack-integration', function () {
         let validUrls = [
             'https://hooks.slack.com/services/;alskdjf',
             'https://hooks.slack.com/services/123445678',
-            'https://hooks.slack.com/services/some_webhook'
+            'https://hooks.slack.com/services/some_webhook',
+            'https://discordapp.com/api/webhooks/380692408364433418/mGLHSRyEoUaTvY91Te16WOT8Obn-BrJoiTNoxeUqhb6klKERb9xaZkUBYC5AeduwYCCy/slack'
         ];
 
         validUrls.forEach(function (url) {