Add new page for sending out invites to employees

[robsmithw]

We need to add a new page for employers to send our invites to employees.

For now, after an employer signs in, it would take them to this page, but in the future, we would want to make sure the employer has paid the premium to use our service.

[robsmithw]

This page should have a basic heading, and for now a button in the top right for "Invite an Employee"

The "Invite an Employee" button should pop up a modal, with a form to include a First name, Last name, and email, and the submit button of the form should be labeled as "Send Invite"