Introduce shared settings

Having a shared place where we can store global commands will make it
easier for any new people joining.  Plus, if anyone comes up with any
good shortcuts, they can share them with others.

Added
rbt-cd (cd to the rbt tree)
rbt-cd-shared (cd to the shared rbt directory)
rbt-cd-shared-settings (cd to the shared settings directory)

Testing:
Started the VM, opened a terminal, ran rbt-cd, rbt-cd-shared, and
rbt-cd-shared-settings. They all worked fine.