Use background tasks for long-running queries

[skearnes]

Adds a couple of new API endpoints:

• /api/submit_query takes the same GET parameters as query and returns a task ID
• /api/fetch_query_result takes the task ID as a GET parameter and returns either a 102 status (processing) or the query result

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.33858% with 11 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@a3e16e5). Learn more about missing BASE report.

Files	Patch %	Lines
ord_interface/celeryconfig.py	0.00%	6 Missing :warning:
ord_interface/api/search.py	94.11%	4 Missing :warning:
ord_interface/api/queries.py	96.15%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #128 +/- ## ======================================= Coverage ? 80.61% ======================================= Files ? 17 Lines ? 980 Branches ? 0 ======================================= Hits ? 790 Misses ? 190 Partials ? 0 ```

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[skearnes]

@miori-nd this has been deployed to staging; see https://staging.open-reaction-database.org/api/docs

[skearnes]

(Update: fixed a connection issue; works on staging now and not just in tests.)