Add indexes to database scheme

[dippindots]

Fix cBioPortal/icebox#469 cBioPortal/cbioportal#10411

This pr fixes two things:

1. The scheme is out of sync between cgds.sql and migration.sql, sync it again
2. Put a limit on VALUE field in clinical_event_data table to avoid the key overlength issue (max key length is 3072 bytes, for MySQL 8 default character set utf8mb4, max characters length are 768)