Improve logging when changing transcripts mapped to a certain variant.

[ifokkema]

Is your feature request related to a problem? Please describe.
Currently, when managing the transcript mappings for a variant, the resulting log entry doesn't describe the changes. This makes it hard to see what happened exactly. Recently, some mappings were lost and had to be restored from backups. From the logs, we can't reconstruct how this has happened, so improved logging is needed.

Describe the solution you'd like
Logging similar to the CuratorAuthorize and CuratorSort actions would be useful.