Refactor: Modularization of Task Scheduling

[GODrums]

Motivation

For future features (such as the leaderboard leagues) we need to be able to execute tasks easily at the end of each leaderboard cycle.
Implements task 3.1 of #170.

Description

This PR reworks the original task scheduler used for the weekly Slack message to be more modular (separating the logic) and easily support adding new tasks:

• LeaderboardTaskScheduler now manages all tasks and schedules them for the end of a leaderboard cycle
• SlackMessageService is now only a light wrapper around the SlackAppConfig, providing implementations for sending messages and testing the connection
• SlackWeeklyLeaderboardTask is a runnable task, scheduled by the LeaderboardTaskScheduler, responsible for creating the Slack message and delegating the sending process

Testing Instructions

No manual testing required - Code reviews would be great though!
Would love to receive feedback on:

• folder structure
• potential for structural simplifications
• does the code need more comments?

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
• Screenshots have been attached (if applicable)
• Documentation has been updated (if applicable)

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 also looks good to me, folder structure and documentation seems also fine to me 🤔