[server] Block changing thread's type to CHAT_SECRET when it has a source message

Test Plan:
I modified thread name update logic on the front end to also change thread type to secret.
Tried to update a subthread created from a sidebar, and it failed as expected.
Tried to update a subthread without a source message, and it was successful.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1003