This PR conditionally runs the CSM BIGINT migration on the CSMH_EXTENDED table,
since this is an optional feature and the table isn't guaranteed to exist.
Currently, this issue is breaking provisioning master sandboxes for us. cf. https://github.com/edx/edx-platform/pull/21493#issuecomment-529009456
*Sandbox URL*: TBD - sandbox is being provisioned.
*Merge deadline*: None
1. verify that the sandbox provisions successfully with this branch and
the ENABLE_CSMH_EXTENDED feature is off.
*Author notes and concerns*:
[ ] @bradenmacdonald
[ ] edX reviewer[s] TBD