[Feature request] Do not hardcode the word `Primer` in the header component.

[bolonio]

Context

The theme hardcodes the word Primer in the header before the project short name defined by siteMetadata.shortName. You can see in the header.js component.

It would be nice to have this information in the siteMetadata, so the theme allows a better header component.

Acceptance Criteria

• [ ] A new field is added to the siteMetadata object, graphQL type...etc.
• [ ] The component header.js reads the first part of the header from siteMetadata.