Generated HTML header should include link to bibrec page

[Rossolson]

The page header section should include a clear simple link to the gutenberg.org/ebook/### bibliographic record page.

[eshellman]

Excellent idea.
Header is defined by
ebookmaker.writers.HtmlTemplates.pgheader(dc)

[Rossolson]

I started the PR before I saw this comment. Did I put the code change in the right place, or are you recommending a different change location?