Initial import of boostrap.py from blendloft

- Renamed to floored_addon_manager.py

- Script lives in release/scripts/startup so that it's always enabled on startup

- TODO: update contents (naming, documentation, comments, and
  potentially the source URL)

[#73420304]
Update floored_addon_manager top-level docstring

- Simplified user documentation to reflect that manual installation is
  no longer needed.

- Removed reference to old bootstrap name

[#73420304]
Rename bslog to _log

Old name was an abbreviation of [b]oot[s]trap, script is no longer
called that.

[#73420304]
Remove remaining references to old bootstrap name

- The upgrade operator name is now floored.addons_upgrade

- Removed BootstrapPreferences; this script is now a startup script so
  it's not used

- Remove unused global "bootstrapversion"

- Otherwise just replaced bootstrap in various places

[#73420304]