Fixed #19501 -- added Model.from_db() method

The Model.from_db() is intended to be used in cases where customization
of model loading is needed. Reasons can be performance, or adding custom
behavior to the model (for example "dirty field tracking" to issue
automatic update_fields when saving models).

A big thank you to Tim Graham for the review!
Included irc link in paragraph to allow for l10n

Translators may want to change the irc link pointing to a
language-specific channel.
Fixed #22933 -- Deprecated django.template.resolve_variable().