Fix dashboard event caching
Created by: dblessing
Fixes #5828 - "Edits in MR comments not get updated on dashboard"
Problem If a user updates/destroys an issue, merge request or comment, the corresponding event on the dashboard is not updated/destroyed.
Solution After update/destroy for any issue, merge request or comment, ensure that corresponding events' updated_at timestamps are modified so the correct content is displayed on the dashboard.