`DotDict` now supports pickling

Celery task was aborting because its `SpeciesImportEvent` couldn't be unpickled.
That was because its `instance` attribute has a `JSONField`, which uses `DotDict`, which lacked pickling support.
Implement job submission front end

* Make the importer's "list" page use real data (merge OTM1 template with OTM2 prototype)
* Add `start_import_endpoint`
* Submit jobs via AJAX and use returned HTML to update job tables
* When "Import" button pressed, show spinner and disable it
Un-indent template
Rename `list.html` as `imports.html`
Merge pull request #1771 from RickMohr/importer-list-uses-real-data

Importer "list" page now works (for species imports)