Change the way that DataModel#updateSource for dryRun: true updates to instead capture and operate on a clone of the data model to avoid a number of edge cases related to handling of dry run backups and validation of dry-run changes.

[dev7355608]

Example Use Case

Failed joint validation does not prevent Toggle Behavior from being persisted

Reproduction:

1. Create a Region and add a Toggle Behavior behavior.
2. Add the same Region Behavior UUID to both Enable Behaviors and Disable Behaviors.
3. Click Update.
4. Joint validation error pops up.
5. Click Update again.
6. No joint validation error this time. The update is committed to the database. After reloading the behavior ends up in the set of invalid documents.