Make the theme compatible with Nikola v7.

The site now builds under Nikola v7, without any warnings.

Changes tl;dr:

* fixed getnikola/nikola#1318.  you were reading this file in binary mode, which made mako go crazy — I switched to codecs.open() which reads as UTF-8.  (this *related posts* solution is nice, BTW)
* fixed `READ_MORE_LINK` rename warning by doing the upgrade
* removed copies of our stock themes, they also prevented from building (changes in messages files)
* upgraded your theme to be compatible with the updated Bootstrap 3 theme (changes include: bootstrap_helper → base_helper, logo support magic in navbar, <head> in a function)