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.
Expose setSystemValue
Redirect user after clicking on link
Use DI
Merge pull request #10584 from owncloud/simple-wizard-trusted-domains

Add a trusted domain wizard