gulp tasks and nunjucks templating

- reworked nearly all gulp tasks
- added html templating with nunjucks
- replaced some old html files
- started correction of scss and js files according to lint results

source/templates/macros/top-macro.nunjucks

@@ -0,0 +1,5 @@
+{% macro field(name, value='', type='text') %}
+<div class="field">
+  <input type="{{ type }}" name="{{ name }}" value="{{ value | escape }}" />
+</div>
+{% endmacro %}