Add a warning to the DAST project when MRs get too big

What does this MR do?

So that DAST developers are conscious of large MRs, add a warning when DAST MRs get too big.

This MR adds two warnings.

At > 500 lines of code the warning is: This merge request is quite big (more than #{git.lines_of_code} lines changed), please consider splitting it into multiple merge requests.

At > 2000 lines of code the warning is: This merge request is definitely too big (more than #{git.lines_of_code} lines changed), please split it into multiple merge requests.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer