Consolidate get_post_types_names() helper

[ilicfilip]

Implements #433

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[ilicfilip]

Closing in favour of #776, which implements the same consolidation against current develop.

This branch had drifted ~2300 commits behind develop and conflicted in 6 of its 7 files. More importantly, it consolidates in the direction the codebase has since moved away from: it deletes Settings::get_post_types_names() and moves the implementation into Content_Helpers, but develop has since settled on Settings as the canonical home — that's where the newer suggested-task and data-collector classes call, and where the exclusion list has been maintained. Rebasing would have meant reverting that decision.

The underlying issue is still real, so #433 stays open until #776 merges. #776 goes the other way: it points the remaining Content_Helpers call sites at Settings and leaves Content_Helpers::get_post_types_names() as a deprecated shim.