Better ui for file download

core/views/export.py

frontend/package.json

frontend/src/components/export/DataExport.vue

frontend/src/components/export/ExportDialog.vue

frontend/src/components/export/InstanceExport.vue

frontend/src/components/mixins/exportMixin.ts

frontend/tests/unit/components/DataExport.spec.ts

frontend/tests/unit/mixins/exportMixin.spec.ts

frontend/yarn.lock

functional_tests/.gitignore

functional_tests/test_export_modal.py

functional_tests/util.py

util/factory_boys.py