Implement root reducer for Reactification

[IreneStr]

Implement the root reducer for the reactified snippet preview and content analysis.

The root state would look like this:

const rootState = {
    snippetPreview: {
        // Whether or not the mobile preview is active.
        mobileIsActive: true,

        // Whether or not the desktop preview is active.
        desktopIsActive: false,

        // The content of the SEO Title input field.
        SEOTitle: "",

        // The content of the slug input field.
        slug: "",

        // The content of the meta description input field.
        metaDescription: "",
    },
    analysis: {
        seo: {
            byKeyword: {}, 
        readability: []
    }
},

We need the following reducer:

• rootReducer: combines all reducers from yoast-components (for now the snippetPreviewReducer and the contentAnalysisReducer)

[abotteram]

This requires changes to YoastSEO.js. Yoast/YoastSEO.js#1227

[IreneStr]

Fixed by https://github.com/Yoast/wordpress-seo-premium/pull/1409