Issue #2900421 by timmillwood, tim.plunkett, catch, Sam152: Architectural review of the Content Moderation module

Issue #2876419 by Sam152, timmillwood, plach: Review content_moderation module and mark code with @internal where necessary

Issue #2725533 by timmillwood, alexpott, amateescu, webchick, dixon_, larowlan, dawehner, catch, Crell, Bojhan, jibran, Wim Leers, agentrickard, Berdir: Add experimental content_moderation module