Documentation should not reference external BCrypt tool

[AndrewCarterUK]

Currently the documentation suggests using this online tool to generate BCrypt hashes.

Although this tool may have been set up with the best of intentions - we have no way of proving that the operator is not farming the input BCrypt hashes (or that the service has not been compromised).

I think these references in the documentation should be removed, as it is not a good security practice to generate password hashes like this.

[stof]

now that we have the commands to generate password hashes in Symfony 2.7+, we should recommend it instead of suggesting the online tool.

[xabbuh]

I agree with @stof. Additionally, we can add a warning on the 2.3 branch to address the concerns raised by @AndrewCarterUK.