Use 'header' as new component name in tracking instead of 'nav4'

[cemms1]

What does this change?

• Uses "header" to indicate the data-component name within the new masthead component instead of "nav4"
• Uses header : topbar : <link-name> as the prefix for the data-link-name attribute of components within the data-component="topbar"

Why?

Agreed with the Data Journalism team and aligned with apps tracking. The word "header" is much easier to grok without in-depth knowledge of the platform than "nav4".