Organizations: post as an organization (`organizationId` on compose)

[Adron]

Part of #80 — Organizations parity gaps

POST /api/messages accepts organizationId. The Android composer always posts as the user.

• Add an author picker (self / each organization the user may post for) when the user belongs to
  at least one, and hide it entirely otherwise.
• Show organization authorship on the message card so it is unambiguous who posted.

Tests: request carries organizationId; card renders org authorship.

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