chore: improvements on org management

[migmartri]

This patch adds a set of improvements to the membership leaving mechanism, but the more important right now is the ability to auto-select a current org if the user has left another one.

• It removes the CLI requirement that prevented leaving current organizations.
• Makes sure that creating a new default membership unset the previous ones (membership.go)
• Makes sure that an inline cas backend is configured when an user manually creates an organization, before it just worked for the automatically generated ones. (biz/organization.go)
• the CurrentOrg method now makes sure to properly set the currentOrg, before it was doing a fallback to the first one without setting the actual flag. (biz/user.go)
• Lastly, a on delete cascade has been added to make sure that when an organization is deleted, its associated apiTokens are removed too.
  Closes feat: revisit requirements to leave org #544

[danlishka]

LGTM, please change the auto-join to auto-select