Fix line-height for some events for mobile

What does this MR do?

Fix line-height for some events for mobile.

Are there points in the code the reviewer needs to double check?

No.

While .event-title and .event-item-timestamp are tuned for PC in https://gitlab.com/gitlab-org/gitlab-ce/blob/v10.0.2/app/assets/stylesheets/pages/events.scss#L12-25, .event-item-timestamp can be omitted for mobile as it is hidden: https://gitlab.com/gitlab-org/gitlab-ce/blob/v10.0.2/app/assets/stylesheets/pages/events.scss#L200-202

Why was this MR needed?

While line-height is fine-tuned only for PC (!2715 (merged) EDIT: !1239 (merged)), it is not suitable for mobile.

Screenshots (if relevant)

before
after

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added 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

What are the relevant issue numbers?

Closes #38778 (closed)