[WIP] - Make UsersView consistent with other Views (Streams/Topic)

[sumanthvrao]

• Make UsersView a urwid.Frame and RightColumnView a urwid.Pile object. This change would be helpful in future if we decide to add buddy PM to RightColumnView as well.
• Move initializing UserSearchbar into UsersView.__init__. This is the header of a listbox.
• Move update_user_list into UsersView. Use a slightly different logic to set contents in Frame after we update the order.
• Extract build_user_view function in RightColumnView which builds the user button list and returns it. This is called from users_view once during initialization and subsequently from update_user_list at 60-sec intervals.
• An old conditional reset_default_view_users is now removed as we clearly distinguish between init call and subsequent list update calls.
• Keypress logic has been introduced to UsersView. This mimics StreamsView.keypress. SEARCH_PEOPLE AND GO_BACK are handled here.
• The updating call from model.py at 60-second intervals has been modified.

[zulipbot]

Heads up @sumanthvrao, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.