Improved flash messages handling in Twig templates

[xabbuh]

Symfony 3.3 will get a simpler way to deal with flash messages in Twig templates: symfony/symfony#21819

I'm going to work on this in PR #7684

[wouterj]

@Ricknox if you reference this issue number in the PR body, the PR will be referenced in this issue. Which is quite a nice way to indicate that this issue has a PR.

Even more, if you prefix the number with "close/closed/closes or fix/fixed/fixes" (e.g. Fixes #7676) in the commit or PR body, this issue will be closed automatically when the PR is merged.