[ticket/security-161] Add form token to styles management form.

SECURITY-161
Merge pull request #3 from phpbb/ticket/security-161

[ticket/security-161] Add form token to styles management form.
[prep-release-3.1.0-RC6] Update version to 3.1.0-RC6
[prep-release-3.1.0-RC6] Add migration for 3.1.0-RC6
[prep-release-3.1.0-RC6] Prepare Changelog
[prep-release-3.1.0-RC6] Add changelog for 3.1.0-RC6
[prep-release-3.1.0-RC6] Update phpBB Version for RC7 development
Merge branch 'prep-release-3.1.0-RC6' into develop-ascraeus