slack_import: Allow replacing uploaded data export files.

Uppy doesn't allow uploading another file and replacing the exisiting
one in the same session due to `maxNumberOfFiles: 1` set by us.
We don't want to remove this restriction, so only way to allow
replacing files easily is to just resert the current uppy session
after the last upload finishes.

Our backend deletes any existing upload by default. So, this works
as expected on all fronts.

web/src/portico/signup.ts

@@ -426,6 +426,10 @@ $(() => {
             $("#slack-import-start-upload-wrapper").removeClass("hidden");
             $("#slack-import-uploaded-file-name").text(file.name!);
         });
+        // Reset uppy state to allow user replace existing uploaded file.
+        uppy.on("complete", () => {
+            uppy.clear();
+        });
     }
 
     if ($("#slack-import-poll-status").length > 0) {