Remove zero-padding from account slug

[kevinmcconnell]

This shortens URLs in the self-hosted case, where external account ID is typically a small number.

@flavorjones are you aware of any reasons that we still need to keep the padding?

[flavorjones]

I think you're correct, there's no good reason it needs to be (at least) seven characters. That convention was lifted from Basecamp's account slug middleware and at the time we still weren't sure whether we were going to allow custom tenant names or not so I opted for minimal changes. But given where we are now, yeah, let's trim it.