PIM-3037 Load dashboard widgets asynchronously

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | Y
| BC breaks?           | Y
| CI currently passes? | almost
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3037
| DB schema updated?   | N
| Migration script?    | ~
| Doc PR               | ~