MAINT-2155: replace embedded mac-updater.app with a Python script.

Remove mac-updater subtree from viewer source, along with the
update_install bash script that invoked it. Remove all mention of mac-updater
in CMakeLists.txt files and in viewer_manifest.py.
Change Mac update_install bash script references in viewer_manifest.py and in
llupdaterservice.cpp (which invokes it) to new Python update_install.py.
Add update_install.py, messageframe.py (which puts up some Tkinter UI) and
janitor.py (cloned from vita, it's exactly what we need here).