Allow rendering menu in separate plugin

[helderco]

These are some changes that I wanted to make for my project (#1), mainly to allow rendering the menu separately. I also went ahead and moved some of what I had in my overriden templates to the default templates, to have less and more semantic markup.

What can break for existing installations?

• Default markup has changed, so for those who don't have an override, it will have to be done now;
• I found I had to re-save my container plugin;

Appart from that, should work as it has.

Model changes

• When installing the app for the first time, not requiring to override a template, I found it better to have a choice as to printing next/prev links, as well as the menu, because I want it in a sidebar, and be able to add social icons below it.
• I removed the subordinate_page in SectionContainerPluginModel because it wasn't being used anywhere.
• The menu functionality works with any implementation of the abstract container, because I'm checking for the abstract.

  HTML markup changes

• <article> is actually a specific kind of <section> for a "thing" that stands on it's own (like in an RSS feed item). I think it's a more semantic choice;
• <main> should be used for the page's main content, and should only be used once;
• Added an <h2> heading to section titles, if they exist;
• Removed some HTML elements to provide de minimum. It's actually all I need in my project.

In my project I've overriden section-container.html to contain only this line:

{% include 'cmsplugin_sections/section-children.html' %}

That's because I want that default structure in my project's base.html, and still allow a default installation to give all that you need.

Other changes

• Passing strings through gettext to allow for translation.
• I'm sending all sections to the template and let it decide in the menu if it's to be shown or not. In my project the link in the logo wasn't working because my first item isn't in the menu. I use the logo as the first item.

[astaaa]

Django CMS has changed and now treebeard/mp_tree.py function get_descendants has no parameter include_self and without this parameter cms/utils/plungins.py function build_plugin_tree doesn't find plugin by parent_id. I changed get_descendants function in my virtualenv, but it's not the right solution.