Icons and the sort of issue events

- Not every icon is in the center.
- We need test function `marginLeftForIconName` in `issue-event-list-item.component.js` for every icons and update.
- The sort doesn't always obey created time. Maybe caused by #848.
- Some items have invalid `created_at` value.

Start in issue-event-list-item.component.js by locating marginLeftForIconName and review how issue events are rendered and sorted. Add coverage for every icon and investigate the invalid created_at values, possibly in relation to #848. Done means icons are centered and events consistently follow created time.

Written by the indexing model from the issue text.