remove duplicate label
[Reference][Form Types] Add missing "format" option to "datetime" field
Changed extends to implements
Update custom_password_authenticator.rst

Correct me if I am wrong... but because we are passing here ```username``` as first argument to ```UsernamePasswordToken```, when one would like to use method ```getUser()```: 
```
...
if (!$this->container->has('security.context')) {
            throw new \LogicException('The SecurityBundle is not registered in your application.');
        }

        if (null === $token = $this->container->get('security.context')->getToken()) {
            return null;
        }

        if (!is_object($user = $token->getUser())) {
            return null;
        }

        return $user;
...
```
from base ```Controller```, it will fail, to get actually logged user and return null.
Fix code block (minimise horizontal scrolling), typo in yaml
[Cookbook][Assetic] Fix "javascripts" tag name typo
Move note about "javascripts" and "stylesheets" blocks to separate note blocks
ApiKeyUserProvider should implements UserProviderInterface
Fixed YAML code blocks in the book
Fixed (mostly) yaml code blocks

Initial idea was to fix the highlighting. While doing that, others things
also got a fix:

* indenting
* missing formats
* some `...` in XML elements
* missing comments in other formats
some language tweaks (AE, third-person perspective)
Fix YAML syntax highlight + remove trailing whitespace
minor #3497 Fix highlighting (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix highlighting

Initial idea was to fix the highlighting. While doing that, others things
also got a fix:

* indenting
* missing formats
* some `...` in XML elements
* missing comments in other formats

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | partially https://github.com/symfony/symfony-docs/issues/1886

Commits
-------

7c0cf89 Fixed (mostly) yaml code blocks
2a43a97 Fixed YAML code blocks in the book
minor #3499 Fix YAML syntax highlight + remove trailing whitespace (ifdattic)

This PR was merged into the 2.4 branch.

Discussion
----------

Fix YAML syntax highlight + remove trailing whitespace

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets |

Commits
-------

0723078 Fix YAML syntax highlight + remove trailing whitespace
document the ability to configure more than one mailer at the same time
minor #3451 some language tweaks (AE, third-person perspective) (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

some language tweaks (AE, third-person perspective)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

0aad5d3 some language tweaks (AE, third-person perspective)
bug #3456 remove duplicate label (xabbuh)

This PR was merged into the 2.4 branch.

Discussion
----------

remove duplicate label

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets |

Commits
-------

b64d73f remove duplicate label
[Reference][Form Types] Add missing options for "form" type
Improvements after review
Improve wording