Fixed #22924 -- Added a note about MEDIA_URL in templates.

Thanks raphael.herouart at gmail.com
Updated urlize regex following a93ee5112d4

Prevent urlize from turning some.organization, an.intern etc.
into urls. Refs #22941.
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.
Fixed #21832 -- Updated prompt message, tests and docs to show that USERNAME_FIELD supports FK.
Fixed #21832 -- Added a way to create USERNAME_FIELD fk instance at command line.