[MCR] populateContentTables maintenance script
This introduces a maintenance script for populating the tables
introduced by the MCR storage schema, namely:
slots
slot_roles
content
content_models
Per default, both the revision and archive tables are processed.
This script is part of the MCR schema migration: after instructing
RevisionStroe to write both the old and the new schema by setting
$wgMultiContentRevisionSchemaMigrationStage = MIGRATION_WRITE_BOTH,
this script can be used to back-fill the new schema for existing
revisions.
Doing this is a precondition to later setting
$wgMultiContentRevisionSchemaMigrationStage = MIGRATION_NEW to
complete the schema migration.
Bug: T182682
Change-Id: Iecc67c1b8c082be1a1039eeb52e76ad16b965226