[lib] Introduce threadTypes.LOCAL as proxy for COMMUNITY_SECRET_SUBTHREAD
[lib] Introduce threadTypes.LOCAL as proxy for COMMUNITY_SECRET_SUBTHREAD

Summary:
As described in D1217, we currently use `COMMUNITY_SECRET_SUBTHREAD` (née `CHAT_SECRET`) for three purposes. Our new type `COMMUNITY_ROOT` will handle one of them, but we're not yet ready to handle "thick" (AKA local) threads.

In the interim we're going to handle them as `COMMUNITY_SECRET_SUBTHREAD` children of the "GENESIS" `COMMUNITY_ROOT`.

I want to introduce the `LOCAL` name now since I'm going to have to go through every usage of `COMMUNITY_SECRET_SUBTHREAD` to check if it needs to be modified to handle `COMMUNITY_ROOT`. Since I'm going through everything now anyways, I might as well consider `LOCAL` in the process, even if it will be a no-op until we actually introduce E2E and "thick" threads (at which point we'll switch `LOCAL` to refer to a distinct number).

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1227

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