Pad body content.

Add some padding to the body so it doesn't ride the window edge in
smaller layouts.
Remove tags from series layouts.

Tags should only appear on the dedicated content page.
Emit tags on dedicated content pages.

Clean up tag and date formatting on dedicated content pages.
Add baseline tagging support.

Support pages of the form "Entries tagged with 'X'" by creating a new
template, tag.html, and several tag pages. Note that since
Jekyll/Pretzel is a static system, tag pages are static. We'll need to
create a dedicated "page" for each tag, however, this can be automated,
and only needs to be done once.