create internal API for model field validation

[Preston Holmes]

Not to be confused with model instance validation.

Currently a model's definition is validated by admin script commands through a function located in django/core/management/validation.py

This utility checks the sanity and integrity of various field definitions on the model, and outputs errors to stdout for display by the command.

There are useful reasons to have a programatic API to this model definition validation - including during app-loading

There may be other 3rd party uses, but the primary use case and inspiration for this ticket, is to allow this validation to occur at app loading time, rather than syncdb time

[Russell Keith-Magee]

I'm going to call this a duplicate of #16905. It's also related to #8579, and the eternally proposed (but never accepted Summer of Code project)

Needless to say, I'm +1.