[IMPROVE] Major overhaul on data importers (#16279)

app/api/server/v1/import.js

app/file-upload/server/lib/FileUpload.js

app/importer-csv/server/importer.js

app/importer-hipchat-enterprise/server/importer.js

app/importer-hipchat/server/importer.js

app/importer-pending-files/server/importer.js

app/importer-pending-files/server/index.js

app/importer-pending-files/server/info.js

app/importer-slack-users/server/importer.js

app/importer-slack/server/importer.js

app/importer/client/admin/adminImport.html

app/importer/client/admin/adminImport.js

app/importer/client/admin/adminImportHistory.html

app/importer/client/admin/adminImportNew.html

app/importer/client/admin/adminImportNew.js

app/importer/client/admin/adminImportPrepare.html

app/importer/client/admin/adminImportPrepare.js

app/importer/client/admin/adminImportProgress.html

app/importer/client/admin/adminImportProgress.js

app/importer/client/admin/importOperationSummary.html