Phriction - improve workflow for when user entered slug needs to be normalized.

Summary: Encountered this playing with T8402 on my test instance. I think warning the user about adding a trailing "/" is unnecessary so don't do it. I think its confusing to not call out spaces / to lump them in with special characters so call out the sapces.

Test Plan: moved a page around and verified no warning if the slug is missing a "/" as user specified and that a change to spaces is called out

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13316