manager checks: tests + implementation + doc
added release note about deprecating DatabaseValidation.validate_field
changed mysql field validation -- replaced hardcoded set of fields with checking field.db_type()
introduced checks.{register, run_checks} functions

- added warnings support: validate doesn't raise CommandError when there
  are only warnings, introduced color support
- replaced get_validation_errors calls with run_checks
doc -- added section about registering checks