Bug 1717051: Rename "mach_bootstrap.py" to "mach_initialize.py" r=firefox-build-system-reviewers,glandium

We've overloaded "bootstrap" to mean three different things:
* The "standalone bootstrap script": `python/mozboot/bin/bootstrap.py`.
  This is to freshly clone a new repo, then run `./mach bootstrap`.
* `./mach bootstrap`: Install necessary dependencies and set up the
  system for development.
* "Mach bootstrap": do the in-process initialization work Mach needs
  before it can run commands.

By using the term "initialize" instead, perhaps we can remove
ambiguity when discussing Mach.

I'm not attached to the name (or this change at all), but I'm interested
in reviewer thoughts :)

Differential Revision: https://phabricator.services.mozilla.com/D120410