Transform header and footer data functions into data objects

[mdubus]

We currently have every text of the website inside the data folder, separated in different files.

The one for the header is the data/header.js. It exports a function called getHeaderData which doesn't receive any argument. This is legacy code, and we would like to rework it in order to have an object instead.

What we have to do is:

• Transform the getHeaderData function of the data/header.js file into an object
• Rename getHeaderData into headerData in every file that uses it (including the stories folder used by Storybook)

Same thing for the Footer (getFooterData becomes footerData + transform function into object)