Add validation for codespace URL

[testower]

Uttu should check that the XmlnsUrl field in exported Codespace element is valid.

Example of invalid Codespace:

<Codespace id="atb">
    <Xmlns>ATB</Xmlns>
    <XmlnsUrl>
        http://www.rutebanken.org/ns/akt
    </XmlnsUrl>
</Codespace>

Xmlns should in this example be http://www.rutebanken.org/ns/atb

[testower]

With provider management now via frontend (enki) it's not possible to get to this state, so only possible if using the api directly (or inserting in db). Low priority.