Build a more robust non-superuser experience

[sastraxi]

• db.userCaps e.g. { createdb: true, pg_stat_file: false}
  • check these caps wherever we're making calls that require them rather than trying-and-failing
• some sort of warning about extension usage for non-superusers
• remove all unnecessary cases of connectAsSuper (migration stuff mostly)
  • need to check this; might be the reason why we can CREATE EXTENSION -- are perms on tables correct though?