Invites: "Email this person" opt-out when granting access directly #61

Part of #57 — Email invites for lists and documents (invite someone who has no account yet)

Web behaviour: when someone is added to a list (or their role changed) they get an in-app
notification and, by default, an email. An Email this person checkbox can be unticked to
skip the email; the in-app notification still goes out. Whether the email actually arrives also
depends on that person's own Sharing notification settings.

Android's manage-access flow has no such control, so it always takes the default.

• Add the checkbox to the add-collaborator / change-role flow and send whatever flag the API
  expects — confirm the field name against the live request before implementing, as it is not
  in the OpenAPI spec.

Tests: flag reaches the request in both states.

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