Change activeKeyword in store to activeSections

[IreneStr]

Currently, we're storing the activeKeyword in the redux state. However, when we're moving the content analysis to the sidebar, we'll have sections, where multiple sections can be active at the same time:

This means there will be no single active keyword anymore. Therefore, we should change the state to contain activeSections, which should be an array (instead of activeKeyword, which is a string).

Current state:

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

Future state:

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

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

NB: Connecting to the UI is not part of the scope of the issue. This issue is purely about the actions and reducer.