Introduce teams

[bobinette]

Needs:

• [x] Owner on paper
• [x] Teams in db:

  type Team struct {
  ID int
  Members []int
  Admins []int
  }

  • [x] Admins can add new members to a team
  • [x] Give access to a paper to the members of a team, base on canEdit

[bobinette]

Completely handled now with the version of auth in #16