Fix: make generated usernames lowercase by default
Fix: make generated usernames lowercase by default

We should use lower-cased username, because it directly corresponds to
some files Software Carpentry stores about some people - and, as we
know, some filesystems are not case-sensitive.

swcarpentry/amy,swcarpentry/amy,shapiromatron/amy,pbanaszkiewicz/amy,wking/swc-amy,vahtras/amy,vahtras/amy,vahtras/amy,pbanaszkiewicz/amy,shapiromatron/amy,wking/swc-amy,wking/swc-amy,swcarpentry/amy,shapiromatron/amy,pbanaszkiewicz/amy,wking/swc-amy