slack_import: Disable submit button if file is invalid.

templates/zerver/slack_import.html

@@ -83,7 +83,7 @@
                         <div class="not-editable-realm-field" id="slack-import-uploaded-file-name">{{ uploaded_import_file_name }}</div>
                     </div>
                     <div class="input-box">
-                        <button type="submit" class="register-button">
+                        <button type="submit" class="register-button"{% if invalid_file_error_message %} disabled{% endif %}>
                             {{ _("Start import") }}
                         </button>
                     </div>

web/src/portico/signup.ts

@@ -457,6 +457,7 @@ $(() => {
             $("#slack-import-start-upload-wrapper").removeClass("hidden");
             $("#slack-import-uploaded-file-name").text(file.name!);
             $("#slack-import-file-upload-error").text("");
+            $("#realm-creation-form-slack-import .register-button").prop("disabled", false);
         });
         // Reset uppy state to allow user replace existing uploaded file.
         uppy.on("complete", () => {

web/styles/portico/portico_signin.css

@@ -1568,3 +1568,10 @@ button#register_auth_button_gitlab {
         text-decoration: underline;
     }
 }
+
+#realm-creation-form-slack-import .register-button {
+    &[disabled] {
+        opacity: 0.6;
+        cursor: not-allowed;
+    }
+}