You need GitHub account for GitHub access due to legal reasons and conditions of EULA. EULA not only prohibits any 3rd party redistribution of the source, but also any form of publication of source code above 30 lines snippets for documentation and help. To make those condition workable you need to tie Epic account tied with GitHub account in order to see the code. Can’t be really helped ;p
You can download source form launcher, it is installation option in the engine, you actually helpful for debugging together with symbols and VS also will point you to it when you ask for definition or deceleration of any function. If you do any C++ in UE4 aspecially a lot deeper like Slate, it is really recommended to have source code on disk. Remember that you effectively extending engine code anyway, it’s integral part of your game or tool you making.
And it will be worth your time, as SlateViewer indeed show of non-Enigne, Core and Slate-only application, there also more complex UnrealFrontend, there Also BlackProgram which show you most minimal example. Most impotent part are build scripts (*.target.cs and *.build.cs) which need to be configured properly for your program to build properly