[Forwardport] Send order email for Braintree Paypal orders

[dimonovp]

Original Pull Request

#13898

Description

This commits adds a frontend observer for sending new order emails.

Order emails are sent using observers on the
sales_model_service_quote_submit_success event, but only in the
webapi_rest and webapi_soap areas. When customers checkout with
Braintree Paypal the order placement occurs in the frontend area with
a request to Magento\Braintree\Controller\Paypal\PlaceOrder, so these
orders were not sent a new order email.

Fixed Issues (if relevant)

1. Braintree Paypal Method No Order Confirmation Email Sent #13778 Braintree Paypal Method No Order Confirmation Email Sent
2. [2.1.10] No order confirmation email after paying with PayPal Express #12792 [2.1.10] No order confirmation email after paying with PayPal Express

Manual testing scenarios

1. Enable Braintree including PayPal through Braintree
2. Complete PayPal purchase on store frontend. The redirect back to the store from PayPal should be to braintree/paypal/review
3. Completing the order place process should result in final request to braintree/paypal/placeorder
4. Will receive the new order email

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-engcom-team]

Hi @miguelbalparda, thank you for the review.
ENGCOM-1002 has been created to process this Pull Request