Update JS push tutorial to leverage UI for creating App/Variant and Sending notification

https://issues.jboss.org/browse/AGPUSH-854

This PR contains modifications that unifies step of creation of application: 

* Push iOS Tutorial
* Push Android Tutorial

Then, it applies update of JavaScript Tutorial

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All platforms now reuse asciidoc and images defined in `include--register-app.asciidoc`
should probably be ".... look at the Example ...."