Added config file support

[Zabuzard]

What is this?

Implements #100 , i.e. adding support for a configuration file. Replaces the previous args system.

Technical details

The configuration file is using JSON (result of votes in #100 ), using Jackson to deserialize it.

args now expects either no or a single argument (the path to the config file), if none was given, it will assume config.json as path.

Example

For testing it locally, create TJ-Bot\application\config.json (or if running from a jar, put it right next to the jar) with content like:

{
    "token": "<your_token_here>",
    "databasePath": "db/database.db",
    "projectWebsite": "https://github.com/Together-Java/TJ-Bot",
    "discordGuildInvite": "https://discord.com/invite/XXFUXzK"
}

(the wiki will be updated with this information as soon as this is merged)

Singleton?

I went for a singleton to make it easier to use Config. Personally, I would prefer passing it around explicitly though - but I am okay with making an exception for a configuration. If you prefer passing it around explicitly, please tell me, then I will refactor that aspect (be aware that the consequence is populating all your constructors and methods with possibly lots of config arguments then though).

[Zabuzard]

@I-Al-Istannen Please prepare the VPS with the corresponding config files before this is merged, otherwise the bot crashes.

[sonarqubecloud]

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[I-Al-Istannen]

I'd prefer passing it around but I can also live with the singleton here