Add focus keywords to redux state

[IreneStr]

We should add the focus keywords to the redux state. We need this when we're moving the content analysis from the metabox to the sidebar.

Current state:

{
	activeKeyword: "string",
	analysis:
		{ 
			readability: [],
			seo: { keyword1: [] } ,
		},
	marksButtonStatus: "string",
}

Future state:

{
	activeKeyword: "string",
	keywords: [],
	analysis:
		{ 
			readability: [], 
			seo: { keyword1: [] },
		},
	marksButtonStatus: "string",
}

keywords should be an array, because users can have multiple focus keywords in Premium.

NB: adding the Premium keywords to the state is not within the scope of this issue.

Please branch from feature/content-analysis-in-sidebar