[FrameworkBundle] Force users to set "kernel.secret" to something different than default "ThisTokenIsNotSoSecretChangeIt"
merged branch stloyd/feature/kernel_secret_change (PR #6598)

This PR was merged into the master branch.

Commits
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f5290b9 [FrameworkBundle] Force users to set "kernel.secret" to something different than default "ThisTokenIsNotSoSecretChangeIt"

Discussion
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[RFC][BC][FrameworkBundle] Force users to set "kernel.secret" to something unique

Bug fix: kinda*
Feature addition: no
BC break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #6480
License of the code: MIT

This PR is to show different approach for "fix" suggested in #6480, as IMO there is no real point for "yet another listener" =)

This PR also introduces BC break for all users that used default value for `kernel.secret`, but IMO it's worth it.