Switched to bookshelf count in getTotalMembers()

no issue

- tested performance between knex raw, knex `count()` and bookshelf `count()` and found no difference over 1000 iterations of each (each ~19,500ms +- 500ms for 104k members locally)
- switched to using bookshelf as the code is the simplest

core/server/services/members/limit.js

@@ -1,13 +1,10 @@
 const config = require('../../../shared/config');
-const db = require('../../data/db');
+const models = require('../../models');
 const errors = require('@tryghost/errors');
 
 // Get total members direct from DB
-// @TODO: determine performance difference between this, normal knex, and using the model layer
 async function getTotalMembers() {
-    const isSQLite = config.get('database:client') === 'sqlite3';
-    const result = await db.knex.raw('SELECT COUNT(id) AS total FROM members');
-    return isSQLite ? result[0].total : result[0][0].total;
+    return models.Member.count('id');
 }
 
 module.exports = async () => {