Blog page front matter cleanup and simplification

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`)