Added currentCountQuery for emails limit

refs https://github.com/TryGhost/Team/issues/588

- This is a basic implementation which needs a review. Implemented it to fix failing tests in main
- Start date is expected to come formatted for DB's needs

ghost/limit-service/lib/config.js

@@ -9,7 +9,16 @@ module.exports = {
             return result.count;
         }
     },
-    emails: {},
+    emails: {
+        currentCountQuery: async (db, startDate) => {
+            let result = await db.knex('emails')
+                .count('id', {as: 'count'})
+                .where('created_at', '>=', startDate)
+                .first();
+
+            return result.count;
+        }
+    },
     staff: {
         currentCountQuery: async (db) => {
             let result = await db.knex('users')