Adds a feature for managing the page's legacy urls

Every time a user renames the url of a page, a legacy page urls gets created. These urls where hidden to the user. Now the user is able to edit them and even add new ones, what adds some nice SEO related advantages.

Also: Adds some fixes for Javascript error handling. I encountered them while implementing this feature and didn't want to make an extra branch for them, because I depend on them in this feature.