Add Author front matter settings

[z64]

We should add front matter properties that allow setting of an author name and avatar URL to be rendered in the template, preferably with a circular mask.

The author should also show up on the archive (articles listing) page.

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• Author front matter
• Implement into page template
• Implement into archive template
• Author avatars

[z64]

Updated the todo-list with the closing of #3 (thanks @unleashy !)