Send email with details of poll

Adds a "POST /api/mail" command to take an email address and send an
email to the specified address.
Make mail sending private

Adds the email to the Session to send when creating. We don't ever store
the email in the database, as we only need it at the time of creation to
send the email.
Add Session.ManageID

Adds the ManageID to sessions, and updates the email link to use it.
Also stops the email task if no email or password is given, as this
often happens when running the unit tests.