initial changes to add an option to select either context or project as grouping of todos on home or tag page

* new menu item in view menu
* js to toggle view
* store setting in cookie to keep de choice of view persistent
* refactor index and tag page of todos to look more alike
* refactor context and project partials to look more alike
do a big refactor of rendering collections of todos for home page, tag page, project page, context page
remove option to show empty containers and start fixing failing cucumber scenarios
get all scenario's running again, first pass.
rebase to master
refactor done todos view and tests
hide group by project from menu to safely merge refactorings with master