This introduces a list of common (recommended) management commands that

users may want to run when viewing rb-site manage --help. The aim is

to give people a quick reference so they don't just try things from the

full list of management commands (now available via manage list-commands).
These commands are organized by category. They are not automatically

scanned from any installed management commands, but are instead

hard-coded, intentionally. In the future, we'll advertise some commands

from Power Pack in here for import/export, which will make people aware

that this feature exists and is most likely the one they want.

Viewed the help output in all supported versions of Python.
Unit tests pass.