feature(server): Update the invite model with additional accessor functions to check tokens, check whether the invitee exists, and to verify new and existing users.
fix(server): Ensure CSRF tokens are used in DELETE requests.
feature(server): Send invitation emails!  Lots of updates to make it happen.

* Extract template setup into its own module so it can be shared with email generation logic.
* Create invitation-email.js to create invitation emails.
* Use nodemailer to send emails - add three transports - console, sendmail, and SMTP.
* Add some tests.
feature(server): Add invitiation verification routes.

* Simplify InviteTokenModel by using tokenInfo as the universal accessor function.
* Add GET-invitation-token and POST-invitation-token.
chore: Add a snippet about guiding principles on the front page.
Merge branch 'invites'