Make system-generated Plan Snapshots provide Name and Description

[Mythicaeda]

Currently, system-generated snapshots made during the plan branch-merge workflow are unnamed. Since the UI is choosing to display these snapshots, we ought to change this behavior.

Between the names and description, we should provide the source plan's name and the action that triggered the snapshot. The generated name requires some type of UUID to avoid raising issues with the constraint that prevents duplicate snapshot names for a plan (potentially the timestamp the snapshot was taken at). The name generation needs a failsafe -- like "[plan name] at [timestamp] (2)".

We may want to discuss declaring a system user to go in the taken_by field, otherwise NULL is still a good way to identify snapshots not taken by an actual user.