Clear mage-cache-sessid cookie on Ajax Login

[pmclain]

This PR is a port of https://github.com/magento-partners/magento2ce/pull/74

Description

This PR adds code based on Magento\Customer\Controller\Account\LoginPost::execute to clear the mage-cache-sessid cookie on login. This triggers a refresh of the customer data in localStorage. The primary purpose is to make sure the customer cart is properly loaded after a successful login.

Fixed Issues (if relevant)

When a customer with existing cart items logs in using the authentication-popup the mini-cart does not display their cart items after login.

Manual testing scenarios

1. Login in using customer/account/login route
2. Add items to customer cart
3. Logout
4. Add virtual product to cart
5. Open mini-cart and click 'Checkout' to open the authentication-popup
6. Login using the authentication-popup
7. The last item cart item will correctly appear merged with the existing quote items in the mini-cart when the page reloads. (Previously only the items from the guest cart would be visible until another action invalidated the cart data in localStorage.)

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.