Fixed #19925 - Added validation for REQUIRED_FIELDS being a list

Thanks Roman Alexander for the suggestion.
Update submitting-patches.txt

Few quick fixes to the terminology in order to match what is used on the Django Trac at the moment.
Merge pull request #1254 from russkel/patch-1

Updated submitting-patches.txt
Allow build of translated documentation
Update media.txt

Fixed typo: froms -> forms
Replaced a dict emulating a set with a set.
Merge pull request #1256 from krmboya/patch-1

Fixed #20575 -- Typo in docs/topics/forms/media.txt
Fixed #20567 - Documented BoundField.id_for_label.

Thanks littlepig for the suggestion.
Fixed #19875 - Added warnings regarding DEBUG=False and empty ALLOWED_HOSTS
Added runserver validation to detect if DEBUG=False and ALLOWED_HOSTS is empty.

Refs #19875.
Refactored file_storage tests to avoid depending on servers tests.
Added a stealth option to flush to allow cascades.

This allows using flush on a subset of the tables without having to
manually cascade to all tables with foreign keys to the tables being
truncated, when they're known to be empty.

On databases where truncate is implemented with DELETE FROM, this
doesn't make a difference. The cascade is allowed, not mandatory.
Added TransactionTestCase.available_apps.

This can be used to make Django's test suite significantly faster by
reducing the number of models for which content types and permissions
must be created and tables must be flushed in each non-transactional
test.

It's documented for Django contributors and committers but it's branded
as a private API to preserve our freedom to change it in the future.

Most of the credit goes to Anssi. He got the idea and did the research.

Fixed #20483.
Defined available_apps in relevant tests.

Fixed #20483.
Fixed available_apps for selenium tests.

Refs #20483.
Revert "Began implementing a shared set of test models to speed up tests."

This reverts commit 22b7870e40a3ecf022b423de6cd867dcb35a6940.
Reverted the introduction of shared_models.

The recent improvements should make shared_models obsolete.

This reverts commit 1059da8de675442e84381d6366c0be254681753e, reversing
changes made to 4fa7f3cdd9bcf50ec4c7f64a31c1dfc02c375f46.
Refactored ValidationError to allow persisting error params and error codes as the exception bubbles up
Fixed #20199 -- Allow ModelForm fields to override error_messages from model fields