Resolve "Improved visualization of SAST results in MR widget"

What does this MR do?

1. Compares head report with base report in order to show:

• Issues fixed in the current MR
• Issues added in the current MR
• All issues in the report

1. When there is only an head report we show them as added issues
2. Adds a new section to show the full report

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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
• 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 #3995 (closed)