Change back to the old entity field naming scheme

This is a retroactive fix for strange behavior I experienced trying to
wrangle Cyder into treating the 'entity' field specially. I decided it
would more efficient for me to just reuse the behavior that's already
implemented regarding hiding and filling the entity field. (See
cyder.base.views.get_update_form near the assignment of
`form.fields[related_type]`.)
Add South custom field introspection rules
Add migrations for new EAV models
Limit choices
Fix more imports
Remove API (for now)
Remove old validation tests (we have new ones)
Fix almost all the things
Don't validate value if not DHCP option/statement
Fix a thing