Fix Code Review and PR leaderboards: correct ORM queries and field mappings

[Copilot]

Fixes #4569, #4660. Code Review leaderboard displayed "No code review data available!" due to incorrect ORM query using GitHubIssue instead of GitHubReview. PR leaderboard included untracked repositories and had fragile avatar rendering.

Backend Changes

• Added extract_github_username() helper - Safely parses GitHub URLs with domain validation to prevent non-GitHub URLs
• Code Review query: Changed from GitHubIssue.objects.filter(type="pull_request") to GitHubReview.objects with correct field paths (reviewer__user__username vs reviews__reviewer__user__username)
• PR query: Added filters for repo__isnull=False and user_profile__isnull=True to exclude untracked repos and orphaned PRs
• Avatar extraction: Both leaderboards now extract GitHub usernames from URLs for proper avatar rendering

Template Changes

• Field names: Updated from reviews__reviewer__* to reviewer__* to match corrected backend
• Avatar fallback chain: GitHub avatar (if URL valid) → Gravatar (if email exists) → Default → "Unknown User" text
• Styling: Unified to #e74c3c brand color, complete rounded borders on all sections
• Added string_filters templatetag for split filter support

Example

# Before (broken)
GitHubIssue.objects.filter(type="pull_request").values("reviews__reviewer__user__username")
# Returns empty - wrong model, wrong field path

# After (fixed)
GitHubReview.objects.filter(reviewer__user__isnull=False).values("reviewer__user__username")
# Returns actual reviewers

{# Template before #}
{% if leader.reviews__reviewer__user__username %} {# Wrong field #}

{# Template after #}
{% if leader.github_username %}
  <img src="https://github.com/{{ leader.github_username }}.png">
{% elif leader.reviewer__user__email %}
  <img src="{% gravatar_url leader.reviewer__user__email 50 %}">
{% else %}
  <span>Unknown User</span>
{% endif %}

Original prompt

please fix this #4651

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