[lib][keyserver] Introduce new threadPermissions types for reactions

Summary:
I want to allow people to react in announcement channels even though they can't send messages, so I'm introducing a new threadPermissions type.

This diff introduces the type, adds it to all roles except GENESIS members, and adds a new migration to populate it.

Test Plan:
I tested this in combination with the next diff by:

1. Running the migration by restarting the keyserver after introducing it and making sure it runs successfully
2. Testing reacting as a member (non-admin) to a message in a COMMUNITY_ANNOUNCEMENT_ROOT community root. I don't have permission to send messages, but I can react on both native and web. I didn't need to log out / back in on native, which indicates the the updatesTypes.UPDATE_THREAD updates were propagated correctly by the migration

Reviewers: ginsu, kuba, rohan

Reviewed By: ginsu

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7073