functionality to add invitees you want to sit with

[grounded042]

An invitee needs to have the ability to add up to 5 other invitees that they would like to sit next to. What we need is:

• an endpoint to get a list of possible invitees. This list needs two things
  1. the attributes: first_name, last_name
  2. for the id, we need to hash it with a unique key on the backend so that one invitee cannot use the id of another invitee to change their response details
• an endpoint to set the list of people an invitee wants to sit with
• include the list of people in the individual invitee endpoint info