Track migration status of repositories in database

Context

The migration API route will need to track the migration status of repositories to a fairly fine degree.

Proposal

Add additional values to the migration_status introduced in #511 (closed) column to hold the migration statuses, such as:

• native - status of a repository that was originally created on the metadata database.
• import_in_progress - status of a repository that is currently undergoing an import.
• imported - status of a repository that has successfully been imported.
• import_failed - status of a repository that has failed to import.
• pre_import_in_progress - status of a repository that is currently undergoing a pre import.
• pre_import_complete - status of a repository that has successfully been pre imported.
• pre_import_failed - status of a repository that has failed to pre import.

Tasks

Check for the follow states before starting import:

• pre-import is currently in progress !860 (merged)
• pre-import failed !873 (merged)
• import is currently in progress !870 (merged)