shell: Rename the new-style lessons from 'bash' to 'shell'

This is mostly about utilities with a few references to the basic POSIX shell,
and hardly at all about Bash itself (as distinct from POSIX shells more
generally).  'shell' is also consistent with the existing:

* _includes/guide-shell
* lessons/ref/shell.html
* lessons/swc-shell
* lessons/thw-shell

As requested in #221 [1].  This conflicts with some of the #221
changes though, so either this PR or that one will have to be rebased
(or otherwise integrated) with the other.

[1]: https://github.com/swcarpentry/bc/pull/221#issuecomment-31959730