Add validation for Old and New URL in both the interface and CSV import

[GeekInTheNorth]

See #174

Add validation to ensure Old Url and New Url are valid absolute or relative Uris Updated the CSV import functionality to skip entries where either the Old or New Url is not a valid absolute or relative Uri.

[GeekInTheNorth]

@valdisiljuconoks Assuming you're okay with the initial approach, I can refactor the model validation into an extension method. I can also then apply this to the TXT and XML imports as well.