Improve community contribution reports

What does this MR do and why?

Community Contributions Report:

• Make MR links clickable
• Display days since last update instead of age for MRs
• Separate MRs into new unique monthly contributors
• Add description and expected next steps for assignees
• Use idle label for filtering MRs

Merge Requests Requiring Attention Template:

• Use idle label for filtering MRs
• Regenerate templates

StaleResources helper:

• Make StaleResources#days_since_last_human_update public so it's usable elsewhere
• Add tests for StaleResources#days_since_last_human_update

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review