Task to force-process offline entities in submission backlog

[ktuite]

Closes getodk/central#682

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]

TODO as of aug 16

• add tests of delay logic vs. force flag for only processing submissions held for a certain amount of time
• change duration to be configurable in minutes for QA?
• try it out more
• add task to central cron job

[ktuite]

Closing this PR in favor of restructured/split PR pair:

#1171 (clean up code, add other useful logic)
and
#1172 (add task to force process)