[server] Split out permissions for CHAT_NESTED_OPEN
[server] Split out permissions for CHAT_NESTED_OPEN

Summary:
Right now we use the same role permissions blob for `CHAT_NESTED_OPEN` and `CHAT_SECRET`. In fact, the same role permissions blob is used for four distinct kinds of threads right now:

1. Communities, which are now handled by `CHAT_SECRET`, but will soon have their own thread type whose role permissions blobs will be created by `getRolePermissionBlobsForOrg`
2. "Thick" threads, which are root (no parent) threads that don't have a hosting keyserver. These too are currently handled by `CHAT_SECRET`
3. Secret subthreads in a community, which are also currently `CHAT_SECRET` (sigh)
4. Open subthreads in a community, which are currently `CHAT_NESTED_OPEN`

As the first step in separating out the thread types and role permissions blobs for these different scenarios, I'm handling the easy one: open subthreads in a community. They already have their own thread type, so I just needed to define their role permissions blob.

Test Plan: Will be tested in combination with following diffs

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D1208

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