Path field needs better help text and validation

Issue Summary

The Path field when setting up a path alias has no validation on it. Additionally the help text states "do not end paths with slash", leading some users to think that the path should then start with a slash. If the user does start the path with a slash, such as "/example", they get redirected to "http://example". Now it's extremely difficult to get back to the page, since all links to it are pointing to the wrong location.

Users might also enter in a Full URL into the path field, ending up with really crazy long aliases that don't make much sense.

We should improve the validation and help text of this field.