Force-process entity submissions held in backlog

[ktuite]

Main PR to close getodk/central#682 following PR #1171

What has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced

[ktuite]

Thoughts on force-processing when entities can be soft-deleted:

• Don't force-apply things in the backlog that are about soft-deleted entities.
• Check what happens with soft-deleted submissions (which can happen today via form-purging)
• Consider deleting things from the backlog when the entity that they reference are purged. (When entity purging happens)