Generate unique slug for the group when provided group slug exists

[Mamaduka]

@boonebgorges update the logic as you suggested.

• AJAX error now returns a unique slug. This is used to replace the suggested one.
• There 5-second delay and if a user starts typing new slug. Replacement action will be canceled.

Closes: cuny-academic-commons/commons-in-a-box#210

[boonebgorges]

Thanks for working on this, @Mamaduka.

I find the 5 second delay to be a bit confusing, as a user. It only takes a second or so for the AJAX request, at which point the 'That URL is already taken' notice is already shown. It's only after a few seconds that the new suggestion is popped into the box, without any indication that it's happening. I understand as the developer that the delay is to allow me time to begin making my own adjustments, but that doesn't feel clear as a user.

Maybe we can rethink this a bit. Instead of replacing the slug after 5 seconds, how about:

1. The AJAX request sends back a unique URL, as you've written it.
2. Instead of showing the generic 'That URL is already taken', have a message like That URL is already taken. [Use "foo-2" instead] or enter a new URL. The bracketed text would be styled as a link, and when clicked, it'd do the replacement.

In other words, we require manual intervention from the user before doing any replacement. @bree-z @jennaspevack what do you think about this approach?