Organizations parity gaps #80

Where this stands

:feature:organizations covers list, create, detail, members (add/update/remove) and user search.
/help/organizations documents three things that have no Android equivalent: joining an
organization, leaving one, and posting as an organization.

Gaps

• POST /api/user/organizations is unused — confirm its semantics (join? create-under-user?)
  against the live API; the spec gives no summary.
• Leaving an organization has no Android affordance.
• POST /api/messages accepts organizationId, which the composer never sends — so an
  Android user cannot post as their organization.
• LinkedIn company pages are covered in the social-linking epic.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.