Shorter user slugs (if possible)

Closes #4211
- Quote John: "In an ideal world, trim at the first space character - so if john is available is a slug, I get that - if not, it tries john-onolan - and finally if that's not available, it tries john-onolan-1 (like normal)". 
This seems like a fragile way to see if we're working with a user slug.  We already have `baseName` from above which is going to be equal to `'user'` for user slugs.