Moving handlers/admin.py to a shiny new python package.

Move notification stuff to a new module, which seems the easiest thing to split
at the moment.
Move handlers/field.py to the more appropriate handlers/admin/field.py.
Keep all the old stuff in admin/__init__py. Sorry for namespace pollution.
What the fuck - simplify custom_homepage assegnment.

Exceptions shall never pass silently.
Simple is better than complex.