Minor fixes mostly around certificates

Hi @bradleybeddoes, @smangelsdorf 

I have four minor fixes - I didn't want to squeeze them all into #159 so they get a separate pull request (but not a separate pull request for each :-)

The fixes are:
* a typo in an error message in CA Usage
* CA usage: skip disabled certificates
* Apply the fix I did in #154 / 8a032c also to the CryptoService issuer / subject methods - makes DNs display nice in the validator output in the GUI.
* Fix broken weblinks in the REST API

Please let me know if you're happy with accepting these changes.

Cheers,
Vlad