Fixed slack setting migration to follow idempotance rule

refs https://github.com/TryGhost/Ghost/pull/12631

- Initialy writen migration would break if executed twice
- Also accounted for situation if only part of the migration executed leaving only one of the values migrated

core/server/data/migrations/versions/4.0/16-refactor-slack-setting.js

@@ -1,7 +1,29 @@
 const ObjectId = require('bson-objectid').default;
 const {createIrreversibleMigration} = require('../../utils');
+const logging = require('../../../../../shared/logging');
 
 module.exports = createIrreversibleMigration(async (knex) => {
+    logging.info('Populating slack_url and slack_username setting values');
+
+    const slackURLSetting = await knex('settings')
+        .select('value')
+        .where({
+            key: 'slack_url'
+        })
+        .first();
+
+    const slackUsernameSetting = await knex('settings')
+        .select('value')
+        .where({
+            key: 'slack_username'
+        })
+        .first();
+
+    if (slackURLSetting && slackUsernameSetting) {
+        logging.warn('slack_url and slack_username setting already exitst');
+        return;
+    }
+
     const slackSetting = await knex('settings')
         .select('value')
         .where({
@@ -28,6 +50,9 @@ module.exports = createIrreversibleMigration(async (knex) => {
 
     const now = knex.raw('CURRENT_TIMESTAMP');
 
+    if (!slackURLSetting) {
+        logging.info(`Populating slack_url setting with value: ${slackUrl}`);
+
         await knex('settings')
             .insert({
                 id: ObjectId.generate(),
@@ -41,6 +66,10 @@ module.exports = createIrreversibleMigration(async (knex) => {
                 updated_by: 1,
                 updated_at: now
             });
+    }
+
+    if (!slackUsernameSetting) {
+        logging.info(`Populating slack_username setting with value: ${slackUsername}`);
 
         await knex('settings')
             .insert({
@@ -55,6 +84,9 @@ module.exports = createIrreversibleMigration(async (knex) => {
                 updated_by: 1,
                 updated_at: now
             });
+    }
+
+    logging.info(`Removing slack setting`);
 
     await knex('settings')
         .where({