Use block templates [1] for all of the full-page templates such as
index.html, _default/list.html, etc. This reduces code duplicated
between them.
Also, move the code in partials/page.html and partials/post.html
to page/single.html and post/single.html respectively. With block
templates the contents of the partials is almost exactly the same
as the contents of the single page templates, and having dedicated
single-page templates for the content types means that we can get
rid of the if/then/else logic in _default/single.html.
This also gets rid of an excess container div; the duplication of
the divs was being obscured by the way _default/single.html was
interacting with the partials. (This is the same duplicated div
as in pull request #51 found by @1138-4EB.)
[1] https://gohugo.io/templates/blocks/
