[native] Only show CommunityDrawerItem for communities

Summary:
We already do this on web, since there we render a separate React component for community items (CommunityDrawerItemCommunity) than channels (CommunityDrawerItem).

On native we use the same component. We shouldn't be showing any options for channels, but we currently show an option to admins to manage the roles. Roles should only be managed on a per-community basis, so this diff hides the "three dots" button for all channels. This results in a much cleaner look with fewer "three dots" buttons.

Test Plan: Confirm that the "three dots" button only appears for community roots, and not for channels on my local iOS simulator

Reviewers: inka

Reviewed By: inka

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12234