Move repeated details to variables _config.yml and general cleanup

_includes/base.html

@@ -0,0 +1,6 @@
+{% assign base = '' %}
+{% assign depth = page.url | split: '/' | size | minus: 1 %}
+{% if    depth == 1 %}{% assign base = '.' %}
+{% elsif depth == 2 %}{% assign base = '..' %}
+{% elsif depth == 3 %}{% assign base = '../..' %}
+{% elsif depth == 4 %}{% assign base = '../../..' %}{% endif %}