Remove automatic favicon generation.

The Grunt plugin seems unmaintained, which is bad since favicon
formats change all the time on mobile devices. Thus, we'll now
fall back to the simplified version of including a touch icon and
a favicon (which will have to be generated manually).

– Remove favicon source and favicon task.
– Add new touch icon and favicon boilerplates.
– Add touch icon and favicon to copy task and head partial.
– Adapt Gruntfile and document changes.