Eliminate hard-coded `import_type` values

* Use `TreeImportEvent.import_type` instead of `"tree"` and `SpeciesImportEvent.import_type` instead of `"species"`
* Simplify related functions
Implement actions in importer "Merge Required" table

* Clicking a row reveals merge controls
* Clicking row again or hitting `Hide` button hides merge controls
* Clicking "new species" or "merge" button posts selected radio choices, updates import row, and reloads status tables (since
  the merged row has likely moved to a different category)
* Disable buttons and show spinner after posting data
* Allow loading status tables with an "active panel" (e.g. Merge Requests)
Merge pull request #1794 from RickMohr/merge-species-js

Implement actions in importer "Merge Required" table