Replace HTMLEditorSanitiser with symfony/html-sanitizer

[GuySartorelli]

We currently maintain a HTMLEditorSanitiser which is mostly responsible for two things:

1. Ensuring only the markup allowed by the HTMLEditorConfig is included in the output
2. Ensuring the output doesn't include XSS or other vulnerability attacks that can be included in HTML markup

Symfony have a symfony/html-sanitizer component that can do the same job, with the benefit that we don't have to maintain our own sanitizer and try to keep it up to date with all possible attack vectors.

See the docs for that component for more details about its usage.

Acceptance Criteria

• Replace as much logic as we can from HTMLEditorSanitiser with Symfony's HtmlSanitizer