migrator: make --forceMigratingTags do the whole migration again

Summary:
Hopefully nobody will need this, but it was useful for me after changing
the case of the properties.

Test Plan: zanshin-migrator --forceMigratingTags, now my contexts have tasks again

Reviewers: ervin

Reviewed By: ervin

Differential Revision: https://phabricator.kde.org/D19239