Add automatic fetch of license-key

[felixboehm]

@hurradieweltgehtunter please share concept here.

[hurradieweltgehtunter]

Please see concept for marketplace side here: https://github.com/owncloud/marketplace/issues/409

If oc instance is connected to the internet, user gets redirected from marketplace to his owncloud instance, api key is included as url parameter.

ToDo's for market app:

• [ ] detect if url contains api key (cloud.mycompany.com/apps/market?apikey=12345abcde) :exclamation: Make sure, ownCloud core preserves url parameters after login. If user is not logged in he gets redirected to ownCloud login page :exclamation:
• [ ] store api key
• [ ] request demo key
• [ ] if available, start downloading/install apps

Please make sure, that the user gets always notified about what's happening. Users can be very sensitive about automated changes in their personal environments. Also, after the install is complete, show a hint á la: "Congratulations. The install is complete. You can now test the ownCloud enterprise Edition for 30 days. If you have any technical questions about apps contact us here. If you want to talk to a sales representative contact us there."