chore: handle (pre)import canceled status in the import handler and importer

Second part of #527 (closed) where we initialize a ticker to check every 1s if the repository has been manually canceled by checking the migration_status column. This column is updated by calling the DELETE import endpoint.

If it the status is canceled, we simply call the cancel context function inside the import go routine.

Builds on top of !935 (merged)

Related to #527 (closed)