Separate header and footer HTML from Content

I saw this done with Starkers and have used it ever since, and always
make this change when starting with JointsWP.

I separate the header.php file into its the vital HTML parts that don’t
change and the content markup parts like the opening DIVs and the logo
and menu.

This way I can easily make custom headers or footers for different
pages (and call them using get_template_part) without having to repeat
the HTML parts, which I can change from the single header-html.php file.
Move sidebar width to pages

Moving the sidebar width out of the sidebar file and into the pages
makes it easier to customise the width of the sidebar according to the
width of the main content.

I can understand that having the columns class on the sidebar.php makes
it easier to change the size of the sidebar as a one off, but because
each of the main-contents would need to be edited anyway it makes more
sense to have them on the same page.
Merge branch 'sidebar-edit' of https://github.com/italiafirenze/JointsWP