settings: Add display setting for demoting inactive streams.

This adds a setting to control Zulip's default behavior of sorting to
bottom and graying out inactive streams.  The previous logic is still
the default "automatic", but this gives users more control.  See the
models.py comment for details.

Fixes #11524.

frontend_tests/node_tests/ui_init.js

@@ -121,6 +121,7 @@ zrequire('starred_messages');
 zrequire('user_status');
 zrequire('user_status_ui');
 zrequire('ui_init');
+zrequire('settings_display');
 
 set_global('$', global.make_zjquery());