Fix SQL Server migration issues

[labkey-adam]

Rationale

https://github.com/LabKey/kanban/issues/1577

Several inconsistencies in the migiration code were uncovered when migrating TNPRC's SQL Server database to PostgreSQL:

1. Linked schemas no longer resolve
2. No issues.Issues rows are present
3. Dataset, issue, etc. table names appear to be empty in the source DB
4. New dataset tables are missing some columns.

Changes

1. Hoist GuidMapperColumn into API. Use it to lowercase linked schema GUIDs that are stored in the ExternalSchema.DataSource column.
2. Don't delete issues, comments, and related issues if no filter is in place
3. Correct scope used in DbSchemaType.Migration
4. Stop stashing the shared container in OntologyManager. A stale value left from pre-migration time caused domains with any shared property to lose all custom properties.