[#12048] Fix course entity migration script

[yuanxi1]

Part of #12048

Outline of Solution
Trying to test the whole migration scripts. Here are some questions and issues found:

• Missing call to verification script [fixed in this pr]
• new students are not added to the entitySavingBuffer [fixed in this pr]
• Seed for deadline extension and instructor is missing
• Seed for account’s google id and student’s google id does not match (email also does not match, which is needed because in the current implementation of how migrated accounts are retrieved based on email), hence whether account is properly linked is not yet tested (fixed in [#12048] Fix seed, migration and verification script for course verification #13086)
• Verification fails, yet to investigate the reason. (by manually query the sql db, seems like the entities counts are correct) - (partially fixed in [#12048] Fix seed, migration and verification script for course verification #13086)
  Questions:
• In DataMigrationForCourseEntitySql::migrateUserAccounts,
  • should we use googleId to query the existing migrated account for students or is email reliable enough
• Added a function in SeedDb.java to clear the datastore for reseeding

[NicolasCwy]

Partially fixed in #13086. The changes in this PR have also been merged there, will close this PR