Add elemMatch support to mongo helper

[bufke]

Description

Adds $elemMatch support to form data api filtering

Implementation

• Add $elemMatch support to mongo helper. Considering what we already allow, this is probably safe. However any modification of a query param to mongo workflow merits a higher level of security review as we could potentially allow injection attacks 🚨
• Handle mongo operation failure on data api with fallback.

Use Cases this solves

• I want to filter submissions to only show results for a specific question inside of a repeating group (which is an array).
• I maybe want to do this for a future gallery update Feature: New Gallery #1349

Other thoughts

• What do you think of my handling of two semi related but not 100% related issues in one PR? It felt small enough to group
• Thoughts on my doing some very very mild refactoring while in the area? Technically removing an unused import isn't 100% guaranteed to be safe in case something is importing *. But we only do that in settings.
• I decided to catch the OperationFailure, check if it was some error I know about, and if not raise a warning and give a more generic message. I didn't want to display any messages right from mongo in case there is some way to force mongo to give out unwanted data via exception message.
• I'm only 80% sure we need elemMatch for future gallery update, but it seems good to support it as a standard alone addition to the data api.

Related issues

Internal bug tracking: KPI-BACKEND-232K

[bufke]

This is pretty bad but the alternative seems to be writing two 95% identical copies of the test for each api version which I think is worse.