overwriting a subject or session clears out IDs of child objects

[ebrahimebrahim]

Overwriting a subject clears out the session IDs in the sessions.json file, creating an invalid database where the sessions.json shows an empty list of IDs while the session file data is left sitting there inaccessible by Database.

Similarly, overwriting a session clears out the solution IDs and the run IDs in the solutions.json and runs.json files.