People: Send invites from section header

[roccotripaldi]

This PR adds an invitation form, which can be toggled from the section header of the Team section.

To test:

• Checkout this branch and make run
• Go to calypso.localhost:3000/people/team/$site where site is a wpcom site ( not currently supported for Jetpack sites )
• In your JS console run localStorage.setItem( 'debug', 'calypso:invite-people-form' );
• Reload the page
• Click the Plus Icon in the section header
• Try sending out some invites, and note the messages in your console

Still ToDo ( handle in an other PR )

• handle success and error notices after clicking 'Invite'
• support adding a custom message via modal
• add to the section header for the other sections, 'followers', etc.
• decide what happens when you click 'People > Add' in the sidebar
• fine tune the design to be closer to Rick's mocks

CC @ebinnion @lezama @rickybanister

[ebinnion]

I think the display name should be PeopleSectionHeaderInviteForm and the class should be people-section-header__invite-form. This is a bit awkward since it's a separate component, but it's meant to be used as a child of people-section-header.

[ebinnion]

Closing in favor of #3058 for now.