Handle empty event timestamp and larger memory units

What does this MR do?

This MR fixes two issues;

• #4653 (closed): Uses numberToHumanSize to convert bytes into appropriate memory units.
• #4664 (closed): Shows Not available when event doesn't have timestamp present

Screenshots

Memory Size

Event status empty

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests updated for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• [ ] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4653 (closed)

Closes #4664 (closed)