Add additional validation rules for slugs, including lower case and valid characters

[ShiftaDeband]

This pull request adds a feature that may be helpful for some: additional validation rules for the slug field.

In our instance, we have some users that are trying to add things like spaces or other weird characters rather than generating the slug with the button. While we are actively trying to not have that happen, we figured it may help if there was a validation rule to do this.

Another issue is that we're trying to enforce lowercase routes. While we can likely do this later in the build process, it may just help to enforce it here as well.

That's how this popped up. We're open to any feedback or anything else that should be done here. Really appreciate the time you've all put into this. It's very helpful!

[mariusGundersen]

Instead of just two hardcoded rules, why not supply a function that can be combined with the existing validation? That way anyone can supply their own validation rules.

[ShiftaDeband]

Instead of just two hardcoded rules, why not supply a function that can be combined with the existing validation? That way anyone can supply their own validation rules.

It looks like that's what you're solving for in part over here, right? I like your approach much better.