[meta] Refactor and remove as many SafeMarkup::set() calls as possible

Follow-up to #1825952: Turn on twig autoescape by default

Problem/Motivation

SafeMarkup::set() is mostly for internal use only. For the most part, existing APIs like t(), String::checkPlain(), XSS::filter(), drupal_render(), etc. should be marking the things they sanitize, and markup in general should be moved into templates wherever possible so the themer has control of it.

#2280965: [meta] Document or remove every SafeMarkup::set() call is postponed on this issue's progress.

Proposed resolution

Remove as many SafeMarkup::set() calls from core as possible.

Remaining tasks

TBD