[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
[Security] Use hash_equals for constant-time string comparison
[Security] Add more tests for StringUtils::equals
[FrameworkBundle] Remove invalid markup
bug #11897 [FrameworkBundle] Remove invalid markup (flack)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #11897).

Discussion
----------

[FrameworkBundle] Remove invalid markup

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11896
| License       | MIT
| Doc PR        | n/a

Commits
-------

1fe8e31 [FrameworkBundle] Remove invalid markup
bug #11374 [DI] Added safeguards against invalid config in the YamlFileLoader (stof)

This PR was merged into the 2.3 branch.

Discussion
----------

[DI] Added safeguards against invalid config in the YamlFileLoader

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11333
| License       | MIT
| Doc PR        | n/a

Exceptions explaining the mistake are better than fatal errors or weird notices appearing when trying to deal with such invalid data.

The XML file loader is not affected by this because the data are validated with the XSD before being processed

Commits
-------

5183501 [DI] Added safeguards against invalid config in the YamlFileLoader
minor #11822 [Security] Use hash_equals for constant-time string comparison (again) (dunglas)

This PR was merged into the 2.3 branch.

Discussion
----------

[Security] Use hash_equals for constant-time string comparison (again)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Use the `hash_equals` function (introduced in PHP 5.6) for timing attack safe string comparison when available.

Add in the DocBlock that length will leak (https://github.com/symfony/symfony/pull/11797#issuecomment-53990712).

Commits
-------

3071557 [Security] Add more tests for StringUtils::equals
03bd74b [Security] Use hash_equals for constant-time string comparison
[SwiftmailerBridge] Bump allowed versions of swiftmailer

Current version of Swiftmailer is 5.2.1, while (previously to this commit)
the version installed by composer was 5.0.3.

This is rather important, since 5.2.1 closes a security issue that 5.0.3 is
vulnarable to (https://github.com/swiftmailer/swiftmailer/issues/494).