Add admin ednpoint for inviting users via email

[picklechips]

Adds an endpoint that does the following:

POST ..../admin/inviteUsers

• Takes users param in the body.

  [{
  name: "Ryan",
  email: "myemail@gmail.com"
  }, {
  name: "Another user",
  email: "anotherEmail@gmail.com"
  }]

• Generates users.length golden tickets on the auth server
• Sends an email to each user, containing a valid golden ticket
• Returns a response which contains a list of errors (if any) and number of successful emails sent.

Requires a gmail email address with less secure apps enabled. Testing via the heroku review app should have the appropriate environment variables.