Rethink default folder icon

The icon we have right now for folders document type might be breaking, at least, two heuristics:

1. Match between system and the real world: we are calling folders to our document types (a physical real world object) and we are not establishing the proper representation;
2. Consistency and standards: it is pretty much a standard for folders to be represented by a representation of a folder icon;

Plus, by having the same icon representing workspaces, sections and templates might cause some confusion to our users.

Even if we know these icons are, most of the times, customised, we should aim to have a good starting point.