[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.
Merge branch 'develop-ascraeus' into develop
[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
Merge branch 'develop-ascraeus' into develop

Conflicts:
	build/build.xml
	phpBB/includes/constants.php
	phpBB/install/schemas/schema_data.sql
	phpBB/styles/prosilver/style.cfg
	phpBB/styles/subsilver2/style.cfg