Create Server Endpoint for the Leaderboard

[GODrums]

Motivation

Closes #77.

Description

• Adds the /leaderboard-endpoint, which provides the required data for the leaderboard calculated on-demand
• Fixes the LazyInitializationException with dynamic eager fetching following Artemis's DB guidelines
• Adds an environment variable monitoring.timeframe to control the number of past days to be fetched for PRs and review comments (creation date)
• Interesting remark: depending on the new timeframe, fetching and hence the server startup can take pretty long. Example for just the Artemis repo: 1 day -> ~20 seconds, 7 days -> ~1 minute

Checklist

General

• PR title is clear and descriptive
• PR description explains the purpose and changes
• Code follows project coding standards
• Self-review of the code has been done
• Changes have been tested locally

Server (if applicable)

• Code is performant and follows best practices
• No security vulnerabilities introduced
• Proper error handling has been implemented
• Added tests for new functionality
• Changes have been tested in different environments (if applicable)

[FelixTJDietrich]

Code looks generally good to me :)