Auto-refresh pending imports

* Factor out `import-tables.html`,  including hidden `importsFinished` field indicating if all imports have finished
* In `importsList.js`, test that field whenver content is loaded, and refresh all tables if imports haven't finished
* New endpoint `refresh_imports_endpoint`
* Refresh every 5 seconds
* Start refreshing when "Bulk Imports" tab is shown, and stop when tab is hidden
* Clean up `import_table.html` and show spinner until import completes