Repair menu highlighting, both for main menu and submenus

During the recent improvements made on the website, the menus and
submenu highlighting for selected menus fell into disarray.
Correction requires changes to the CSS and templates both, because
Django doesn't really have a convenient way to set variables in
templates to use to solve this.

There are still a few submenu items highlighting not working even
after this commit. More work needs to be done.

Absolute URL back to /blog for year.

<tmarble> if you go to http://aspen.sfconservancy.org/blog/
<tmarble> then you click on 2014 in leftnav [15:46]
<tmarble> you get the new leftnav
<tmarble> BUT the URLS are borked
<tmarble> http://aspen.sfconservancy.org/blog/2014/2015/
<tmarble> http://aspen.sfconservancy.org/blog/2014/2014/
<tmarble> etc.

Fix blog by year index.

Add an additional context data that is always the entire list of years,
and use that instead of date_list for the current queryset.

Move blog index to be part of base template.

This way, it's not on the bottom of pages. This shouldn't cause
trouble, since when date_list isn't defined, nothing is shown by this
code.