Optimised email stats aggregation query for typical column usage

ref https://linear.app/tryghost/issue/ENG-790/remove-use-of-sub-queries-in-email-analytics

- the `delivered_at` column is typically entirely/nearly entirely filled with values meaning the `IS NOT NULL` query matches a huge number of rows that MySQL has to fetch from the index to count
- using `IS NULL` switches that behaviour around as it will now match very few rows which has been shown in testing to be considerably quicker
- after switching to `IS NULL` the query returns an "undelivered" count rather than a "delivered" count, in order to keep the rest of the system behaviour the same we can calculate the delivered count by subtracting the query result from the total number of emails sent which we can fetch using a very fast primary key lookup query on the `emails` table

ghost/core/core/server/services/email-analytics/lib/queries.js

@@ -41,12 +41,14 @@ module.exports = {
     },
 
     async aggregateEmailStats(emailId) {
-        const [deliveredCount] = await db.knex('email_recipients').count('id as count').whereRaw('email_id = ? AND delivered_at IS NOT NULL', [emailId]);
+        const {totalCount} = await db.knex('emails').select(db.knex.raw('email_count as totalCount')).where('id', emailId).first() || {totalCount: 0};
+        // use IS NULL here because that will typically match far fewer rows than IS NOT NULL making the query faster
+        const [undeliveredCount] = await db.knex('email_recipients').count('id as count').whereRaw('email_id = ? AND delivered_at IS NULL', [emailId]);
         const [openedCount] = await db.knex('email_recipients').count('id as count').whereRaw('email_id = ? AND opened_at IS NOT NULL', [emailId]);
         const [failedCount] = await db.knex('email_recipients').count('id as count').whereRaw('email_id = ? AND failed_at IS NOT NULL', [emailId]);
 
         await db.knex('emails').update({
-            delivered_count: deliveredCount.count,
+            delivered_count: totalCount - undeliveredCount.count,
             opened_count: openedCount.count,
             failed_count: failedCount.count
         }).where('id', emailId);