[server] Allow creating a personal thread
[server] Allow creating a personal thread

Summary: This is a minimal change which allows creating a personal thread

Test Plan: I sent a message in a pending thread, and new thread was created. Creating new threads by using compose thread still works.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D328

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