refactor: migrate ENABLE_CSMH_EXTENDED off FEATURES-as-dict in the CSM bigint migration

ENABLE_CSMH_EXTENDED is a flat setting (openedx/envs/common.py, default True); its app
readers (courseware/models.py) and tests already use settings.ENABLE_CSMH_EXTENDED. The
2019 0011_csm_id_bigint data migration still read settings.FEATURES["ENABLE_CSMH_EXTENDED"]
in two places:
- the Migration class body, evaluated at import time (on every migrate / makemigrations /
  test-DB build), gating an extra dependency; and
- database_forwards(), gating the studentmodulehistoryextended ALTER TABLE.

Switch both to bare settings.ENABLE_CSMH_EXTENDED, matching models.py. Behavior is
identical, and reading the flat setting keeps the migration working for the deployments
that still apply it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>