fix(calendar(js)): escape CSS selector names

[leonklingele]

SOGo uses the name of calendar categories verbatim to construct a CSS selector without escaping characters such as "/". This patch ensures those selector names are properly escaped so calendar categories applied to a calendar event match the according selector and appear in the correct color.

This patch makes use of the CSS escape API which is supported by all major browsers. See https://caniuse.com/mdn-api_css_escape.

Also see https://mathiasbynens.be/notes/css-escapes.