[#233] Goal setting for users

[wjun856]

• Add the new goal data model
• Add goal to the person schema
• Route GET /goal
  • If there is a goal, return the goal object and the progress
  • If there is no goal, return no content
  • Before it sends a goal, it should check if the end date has surpassed
  • If the date is surpassed and it is a recurring goal, update the current start/end dates accordingly
  • If the date is surpassed and it isn't a recurring goal, remove the goal and return no content
• Route POST /goal
  • Creates a new goal
  • The body will contain: { encounters: number, duration: string, recurring: boolean }
  • Should use the duration string and the current date to compute the start/end date of the goal
• Route PUT /goal
  • Updates the current goal
  • Same body contents as POST /goal
• Route DELETE /goal
  • Deletes the current goal