In the last diff, I introduced a new threadPermissions constant for reactions.

In this diff, I update permission checks on the keyserver and client to check this new constant.

Depends on D7073

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