fix(backup): Fix query logic for dangling model exports

[azaslavsky]

There are a small number of models that have no unambiguous direct connection to their relocation scope's root model - these are called "dangling" models. The key factor that defines them, and makes them difficult to handle, is that we cannot use our "query already exported foreign keys" filtering methodology to select only the models relevant to our export targets, because these models have no foreign keys that connect them back to the root of that target. For example, TimeSeriesSnapshot has no foreign keys at all, see: https://tinyurl.com/27z4x6tk.

In cases like the one above, we ended up exporting ALL of the TimeSeriesSnapshots in the database - clearly a very bad outcome when we only want to export those related to a specific org! A better approach is to define custom filtering logic for these models, thereby enabling them to use "adjacent" models in the model graph to select only models that we care about for a given export. In the example above, we query all Incidents filtered down by our previous exports to get a sneak-peek at the set of IncidentSnapshots (a set that is currently empty due to going in reverse dependency order), then use that information to work backwards to grab the TimeSeriesSnapshots we need.

The upshot is that this commit introduces a generic method for constructing filtered queries for a specific model, the overridable query_for_relocation_export.

Issue: getsentry/team-ospo#203