[native] Add subchannnels modal

Summary:
Linear issue: https://linear.app/comm/issue/ENG-2373/subchannels-view
Adding a subchannels modal, similar to the SidebarListModal that displays sidebars of a chat.
This modal is supposed to display subchannels of a given chat. It is to be used in the community drawer.

As the designs are still being discussed, this diff doesn't introduce the final styling yet. Here is what it looks like for now:

Test Plan:
In chat-thread-list.react in onPressSeeMoreSidebars exchange the navigation call for:

this.props.navigation.navigate<'SubchannelsListModal'>({
      name: SubchannelsListModalRouteName,
      params: { threadInfo },
    });

Run ios simulator, press “See more…” under a chat that has threads and see that the SubchannelsListModal appears

Reviewers: przemek, kamil, tomek

Reviewed By: kamil, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5972