Add a <main> tag to surround all the content blocks, so that it's easier
to target the main part of a page (be it an article, the list of posts
or the different categories) using CSS.
Because of this, the <section> part of the article.html template is made
redundant, so it is removed.
Finally, the generic <div> is replaced by an <article> tag to surround
the article's content.
In the simple theme, some templates are using `h1`, others are using
`h2` for the main title of the page (other than the one in the header).
This commit changes that so all of the pages are using `h1`.
Before this commit, the behavior of the simple theme is to hide the
category and tags if there is no author defined, even if the
category/tags are present. This appears to be unintentional, due to a
misplaced `endif`.
Fix this by moving the endif to the right place, so that the other if
blocks aren't nested.
The pubdate given is in no way an abbreviation. Hence the semantics of
<abbr> do not justify its usage in this context.
<time> was introduced in HTML5. It's datetime attribute can override the
textContent. Anyway it specifies a date+time of a given event.
http://www.w3.org/TR/html5/text-level-semantics.html#the-time-element
Originally the pubdate attribute was meant to annotate that the
publication date of the closest section unit is specified, but this was
dropped and is not part of the official specification.
https://www.w3.org/wiki/HTML/Elements/time
publication time and date and the last modified time and date
independently.
This makes it possible to access the last updated date with {{ article.locale_modified }} in templates.
Additionally, an already delivered feed entry can be corrected by changing the modified date and time, as it is used for atom:update
/ rss pubDate field now.
As it is done when reading the file, we need to remove html tags for the
permalink and the slug (this is done here for the notmyidea and simple themes).
While modifying the themes I also replaced the `pagename` template tag with
`article.url` (`pagename` was an empty variable, no more used ?).
