Allow overriding the subject on the invitation email

[agerbens]

We should be able to override the subject on the invitation email when either:

• Using the invite-email template
  OR
• Passing a template in through the request body.

[spruce-bruce]

This might be doable now - or at least I imagined doing this with the options column in the template table. The idea being that you can set the subject in the table with { subject: 'some subject' }. Updating the invite endpoint to take a subject seems pretty reasonable to handle dynamic subjects, too