Add feature flag for database reindexing

Merged Andreas Brandl requested to merge ab/reindex-cron-prep into master

What does this MR do?

This only adds a feature flag for database index rebuilds.

This way, we can already run with #249662 (comment 415121695) to setup a cronjob triggering this rake task (while we're completing its implementation).

Background on reindexing: &3989

Edited by Andreas Brandl