Update src/Symfony/Component/HttpFoundation/Request.php

This makes `getContentType()` work when a regular form is submitted.
[Form] Choice children can be template customized like collection
[Serializer] Added PropertyNormalizer, a new normalizer that maps an object's properties to an array
Call AuthenticationManager in AnonymousAuthenticationListener
[HttpKernel] Extract method to make callable controller in ControllerResolver

correct naming
[Form][Validator] All index items after children are to be considered grand-children when resolving ViolationPath (fixes #11458)
add allow_html5 option to date and time FormType to disable HTML5 date input when widget is set to single_text

remove unnecessary test when creating datetime format data transformer
[MonologBridge] Ignore empty context/extra by default in the ConsoleFormatter
[WebProfilerBundle] turbolinks compatibility
[Translation] made XliffFileDumper support CDATA sections.
[FrameworkBundle] Add Event Dispatcher debug command
[Form] FormBuilder::getIterator() now deals with resolved children
[Twig][Form] Moved configuration key twig.form.resources to
twig.form_themes
fix handling of empty sequence items

When a line contains only a dash it cannot safely be assumed that
it contains a nested list or an embedded mapping. If the next line
starts with a dash at the same indentation, the current line's item
is to be treated as `null`.
[Monolog] added NotFoundActivationStrategy from MonologBundle
make the stopwatch service always available

Previously, one had to be careful to check if the debug.stopwatch
service was available before using it. Otherwise, the application
would break in the prod environment.
[DI] Added safeguards against invalid config in the YamlFileLoader

Exceptions explaining the mistake are better than fatal errors or weird
notices appearing when trying to deal with such invalid data.
Closes #11333
updated CHANGELOG for 2.3.19
update CONTRIBUTORS for 2.3.19
updated VERSION for 2.3.19