Fixed batch size when sending bulk emails

- during a refactor, I moved the `BATCH_SIZE` variable around
- putting the variable export above a more general export means it get
  overwritten and the value is `undefined` outside of the module
- when we chunk the emails, we were chunking in sized of `undefined`,
  so I'm guessing it just defaulted to 1
- this means the email batches were of size 1 instead of 1000 - oops

ghost/mailgun-client/lib/mailgun-client.js

@@ -2,8 +2,6 @@ const _ = require('lodash');
 const debug = require('@tryghost/debug');
 const logging = require('@tryghost/logging');
 
-module.exports.BATCH_SIZE = 1000;
-
 module.exports = class MailgunClient {
     #config;
     #settings;
@@ -208,3 +206,5 @@ module.exports = class MailgunClient {
         return !!instance;
     }
 };
+
+module.exports.BATCH_SIZE = 1000;

ghost/mailgun-client/test/mailgun-client.test.js

@@ -25,6 +25,10 @@ describe('MailgunClient', function () {
         sinon.restore();
     });
 
+    it('exports a number for BATCH_SIZE', function () {
+        assert(typeof MailgunClient.BATCH_SIZE === 'number');
+    });
+
     it('can connect via config', function () {
         const configStub = sinon.stub(config, 'get');
         configStub.withArgs('bulkEmail').returns({