Marginalia with feature check on app start

We merged !19759 (merged) and it was reverted !21015 (merged) because of ThreadError: deadlock #37897 (closed) error noticed in Staging when DB Loadbalancing is enabled.

This MR fixes the deadlock by moving Marginalia feature check to be on app start and the result is cached in an instance variable. More details in this thread - !21015 (comment 254137159)

As acknowledged in the comment thread since we have the feature check on app start it would require restart/SIGHUP of Webapp/Sidekiq workers to enable/disable the feature.

Closes #27857 (closed)