[prep-release-3.1.0] Update version to 3.1.0
[prep-release-3.1.0] Prepare Changelog
[ticket/13208] Add security issues to the final changelog

PHPBB3-13208
Merge pull request #3059 from nickvergessen/ticket/13208

[ticket/13208] Add security issues to the final changelog
Merge branch 'prep-release-3.1.0' into develop-ascraeus

* prep-release-3.1.0:
  [ticket/13208] Add security issues to the final changelog
  [prep-release-3.1.0] Prepare Changelog
  [prep-release-3.1.0] Update version to 3.1.0
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/13208] Add security issues to the final changelog
  [prep-release-3.1.0] Prepare Changelog
  [prep-release-3.1.0] Update version to 3.1.0

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