Fixed #21755 -- Added ForeignKey support to REQUIRED_FIELDS.

This allows specifying ForeignKeys in REQUIRED_FIELDS when using a
custom User model.

Thanks cjerdonek and bmispelon for suggestion and timgraham for review.
Fixed #22924 -- Added a note about MEDIA_URL in templates.

Thanks raphael.herouart at gmail.com
Fixed #22915 -- Document backward incompatible changes in the ValidationError constructor.

This patch also fixes update_error_dict to better handle the use case described
in this ticket, previously the type of the provided container could be lost in
some conditions.

Thanks Russell Keith-Magee for the report and Tim Graham for review.