Make it easier to customize Introduction

[jacobwod]

Currently, the steps that are presented by the experimental Introduction tool are hard-coded, which is never good.

In the long run, there should be a GUI in Admin to control what steps are shown.

In the meantime, however, we must allow webmasters to control what elements are being introduced. One way of doing this is by using appConfig.json. As long as this feature is experimental, I feel that it's okay to have it like this.

So, the idea is that in addition to experimentalIntroductionEnabled admin can set another flag: experimentalIntroductionSteps, that will contains an Array. Refer to intro.js-react's documentation.

[jacobwod]

I have also added a Wiki page about configuring this, see https://github.com/hajkmap/Hajk/wiki/Experimental-features#step-by-step-guide-and-feature-introduction-introjs.

[jacobwod]

Done as of e41b6e0602e5433f87d9ebc18f0e4919cb4d2591.