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