LT-15296: Import FlexText Twice

Fix import twice without merge:
- Check for duplicate GUID before creating CmMediaURI's
- Update references thereunto

Fix import twice with merge:
- Maintain existing Media Containers (to prevent orphans)
- Update existing Media URI's

Add and improve unit tests.

Change-Id: I6b4827365dff4f831692993a8e0f9bc5b1d553f6