feature #442 Add a security Post Voter (yceruto, javiereguiluz)

This PR was merged into the master branch.

Discussion
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Add a security Post Voter

Implement https://github.com/symfony/symfony-demo/issues/440 feature.

- [x] Add more code comments and doc references

Following some references and recommendations in:
- https://symfony.com/doc/current/components/security/authorization.html
- http://symfony.com/doc/current/security/voters.html
- https://stovepipe.systems/post/symfony-security-roles-vs-voters (@iltar blog)

Any suggestion is welcome!

Commits
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a592eeac Minor reword in the help note
8468423e Simplified the voter code a bit
c14e9dbc Add security post voter