Blog page front matter cleanup and simplification

[chalin]

There are a few issues with the blog-page front matter attributes and processing. (These are thoughts that arose as I was working on #376.)

• [ ] On the old site, the thumbnail front-matter attribute was displayed on the left of the author's name. For example:

  

  We no longer use the thumbnail attribute, so consider simplifying the entries and dropping the attribute, or update the blog-entry hero.html partial to make use of it.

• [ ] There is a bit of an inconsistency in the design and processing of the author & authors attributes, as compared to the position attribute. For example, there is a separate link sub-attribute associated with the name of an author. In contrast, links for position attributes are embedded using inlined links (in mardown syntax). Since we're applying markdownify to the author's name anyways, we might as well simplify the processing and embed any author name links in the name field using markdown too.

• [ ] Is it really worth having both author and authors fields; the main cost is the complexity of the template (hero.html)