[MCR] populateContentTables maintenance script

[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