Improve content sanitiser

[JamesDPC]

Changes

• Update config: default_allowed_html_tags is now an array
• Remove: getAllowedHTMLTagsAsArray()
• Config: removeFormatPasted is now false (the default)
• Config: resetCss is now false (the default)
• Remove h2 from default tagsToRemove configuration
• Move script loading to a dedicated JS, loading options from a data attribute
• Support CSS.AllowedProperties in santiser, allow configuration to set allowed CSS properties - empty by default
• Allow configuration to set default allowed attributes -empty by default
• Remove tagsToKeep, and use default_allowed_html_tags as source for that Trumbowyg options
• Support custom fields adding Trumbowyg plugins
• Handle empty value to avoid unnecessary operation on empty string
• Update documentation
• Update tests