Add a trusted domain wizard

Adds a little button to the trusted domain warning, if an admin clicks on the warning he will be redirected to ownCloud and asked whether he want to trust this domain.

By far not the cleanest code, or clean at all, but does the job and I don't see a reason to make a lot of changes for this little improvement. I think there is no need in making "yet another" admin setting, automagic is at least preferred by myself ;-)

A check on the wording would be much appreciated.

How it looks:
![screen shot 2014-08-21 at 22 22 56](https://cloud.githubusercontent.com/assets/878997/4002902/69387dec-2971-11e4-9a7f-1fc01e76f99b.png)
![screen shot 2014-08-21 at 22 23 06](https://cloud.githubusercontent.com/assets/878997/4002911/76fc453a-2971-11e4-937d-afd9695e45e7.png)

@MTRichards @craigpg @karlitschek