Adds a new "hidden" argument to mach decorators that allows the command to execute but will not be visible in `mach help`. Removes `mach pastebin` functionality, marks it as "hidden", and displays a notice that pastebin.mozilla.org has been decommissioned. Differential Revision: https://phabricator.services.mozilla.com/D240280
==== mach ==== Mach (German for *do*) is a generic command dispatcher for the command line. To use mach, you install the mach core (a Python package), create an executable *driver* script (named whatever you want), and write mach commands. When the *driver* is executed, mach dispatches to the requested command handler automatically. To learn more, read the docs in ``docs/``.