Developer notifications: Slack bot with weekly post

[GODrums]

Motivation

One of our goals is to integrate Hephaestus into Artemis's workflows as seemlessly as possible. This includes providing developer notifications about results in the leaderboard. As the primary channel for communication in Artemis, we primarily focus on Slack here.

Description

This PR sets up the new Slack-Bot HephaestusBot for usage within our application-server:

• Automatic testing if the bot works on startup (connection, token + auth)
• CRON-job for a leaderboard message composed with Slack's Block Kit
• Env-vars for: disabling the scheduled message, cron schedule, Slack bot token, Slack signing secret, Channel to send messages in
• Leaderboard weeks now start at Tuesday 9am and last exactly 7 days
• Additionally: Adjusts the timeframe-filter of the leaderboard (see screenshot below)

Screenshots (if applicable)

Example of the scheduled Slack message:

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]

Generally looks good but I would not hardcode those values on the client side. Also added some improvement comments. But once they are implemented we can merge this PR.

[FelixTJDietrich]

@GODrums, I tried to improve the consistency of the config names in 8c9fb70

[FelixTJDietrich]

Looks good now, thank you