Add support screen

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

A support form, which will submit a ticket to Zendesk.

How to test the changes in this Pull Request:

1. observe that there is no "support" submenu item in Newspack menu in admin
2. add necessary environment variables to wp-config.php:

define('NEWSPACK_SUPPORT_API_URL', 'https://my-super-support.zendesk.com/api/v2');
define('NEWSPACK_SUPPORT_EMAIL', 'support@site.com');

3. observe that there is a "support" submenu item in Newspack menu in admin
4. visit the support wizard, you should see a form
5. fill in subject and message and submit the form
6. a success message should greet you
7. a ticket should appear in the Zendesk instance

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[claudiulodro]

The PHP code looks good and this works as intended. Good work! I'd hold off merging until we finish discussing, though. :)

[thomasguillot]

Looks good. I've just added some tiny visual changes in 8e473ae