Resolve "Refactor homepage's work items and MRs widgets"

What does this MR do and why?

• Abstracts MergeRequestsWidget and WorkItemsWidget into UserItemsCountWidget to simplify grid and removing duplicated code
• Consistently using the term work items except for displayed texts and tracking values

References

Refactor homepage's work items and MRs widgets (#572532 - closed)

Screenshots or screen recordings

Viewport size	Before	After
< lg (>=992px)
=> lg (>=992px)

How to set up and validate locally

1. Go to personal homepage
2. Check widgets on different resolutions
3. Make sure issue count on the left bar and issue count in personal homepage are the same

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #572532 (closed)