Add a toast notification on save/delete of an approval rule.
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### Summary
When saving or deleting a Merge request approval rule, we should show a toast notification.
This change should affect all pages where the Approval rule selector is being used:
* **Project \> Settings \> Merge requests \> Merge request approvals**
* **Project \> Settings \> Repository \> Branch rules \> Branch rules details**
* **Merge request \> New/Edit**
issue