feat: expand groups to individual addresses

[madbob]

Considers this a proof-of-concept, as some more work is required including:

• handle contacts' avatars
• suppress the no-longer required GroupsIntegration::expand() function (and adjust TransmissionService)

Fixes #2723
Fixes #9075
Fixes #3132

[madbob]

Any feedback about this approach?

The action to replace a Group selection with the exploded list of recipients involved in that same Group is a viable new behavior for Mail?

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