Support emoji reactions on pull request comments

[Philipp0205]

Summary

Users should be able to add and view emoji reactions (👍, ❤️, 😄, etc.) on pull request comments, similar to GitHub and Bitbucket's reaction features.

User Story

As a pull request participant, I want to react to comments with emojis, so that I can quickly acknowledge or respond to feedback without writing full replies.

Proposed Solution

• Display existing reactions below each comment with emoji and count
• Add UI control (button or menu) to add reactions to comments
• Show which users reacted with each emoji (tooltip or popup)
• Support removing reactions
• Sync reactions with provider APIs
• Common reactions: 👍 👎 😄 🎉 😕 ❤️ 🚀 👀

Acceptance Criteria

• Existing reactions are displayed with emoji icons and counts
• Users can add reactions to any comment
• Users can remove their own reactions
• Tooltip shows which users used each reaction
• Reactions sync with GitHub API
• Reactions sync with Bitbucket API
• UI updates immediately when reactions are added/removed
• Emoji rendering works across different platforms

Technical Considerations

• GitHub: Use reactions API (/repos/{owner}/{repo}/pulls/comments/{comment_id}/reactions)
• Bitbucket: Check if Data Center API supports reactions (may be limited)
• Use Unicode emoji characters or emoji icon set for rendering
• Handle emoji font rendering in SWT (may need emoji font fallback)
• Update PullRequestComment model to include reactions list
• Cache reaction data to minimize API calls
• Consider emoji picker UI component