XWiki Commons missing escaping of `{` in Velocity escapetool allows remote code execution

XWiki Platform is a generic wiki platform. Starting in version 3.0.1 and prior to versions 4.10.19, 15.5.4, and 15.10-rc-1, the HTML escaping of escaping tool that is used in XWiki doesn't escape `{`, which, when used in certain places, allows XWiki syntax injection and thereby remote code execution. The vulnerability has been fixed in XWiki 14.10.19, 15.5.5, and 15.9 RC1. Apart from upgrading, there is no generic workaround. However, replacing `$escapetool.html` by `$escapetool.xml` in XWiki documents fixes the vulnerability. In a standard XWiki installation, the maintainers are only aware of the document `Panels.PanelLayoutUpdate` that exposes this vulnerability, patching this document is thus a workaround. Any extension could expose this vulnerability and might thus require patching, too.