achievements

[hanneskt]

add achievements. this needs:

• have achievements!
  • achievements with multiple goals
• unlocking achievements
  • services can unlock achievements for users with a key
• adding achievements
  • ability to add services
  • each service can add achievements

[hanneskt]

goals should have slugs, so they are clearer for other people

[hanneskt]

• verify sequence is valid
• improve validation logic (for example, achievement must have at least 1 goal)