shim for cross-shard conversations without messageable_users working
shim for cross-shard conversations without messageable_users working

closes CNVS-3557

test plan:
 * view a cross-shard conversation
 * it shouldn't give you a page error

Change-Id: I5100554ba592ef03816757f79b205efa0012c948
Reviewed-on: https://gerrit.instructure.com/17380
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Clare Hetherington <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>

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