Add "Copy all code" button for all code snippets

[PavelRPavlov]

Competition (another design) as well as MSDN docs provide it. Having such button is widely accepted pattern and customers expect to have it. Not having it may lead to unwanted reaction in customers.

[imtodor]

The feature is ready in https://github.com/telerik/docs-seed/tree/copy-code-snippet branch. Please test and let me know if you experience any issues or have feedback to share

[imtodor]

#89