localeCustomStrings has a pretty w.e.i.r.d. lifecycle (involving table `civicrm_word_replacements`
and column `civicrm_domain.locale_cusotm_Strings`).  This commit does not unweird it, but the
weirdness no longer involves special-case logic on `CRM_Core_Config*` -- all the weirdness is
concentrated in `CRM_Core_BAO_WordReplacements` and `CRM_Admin_Form_WordReplacements`.