Auto-prune of full helper roles

[Zabuzard]

Overview

Closes #483. This is a somewhat urgent mitigation due to a technical limitation. Our helper-role system breaks once a helper role has more than 100 users (usually the JAVA helper role).

This mitigates the problem by automatically removing some users from helper roles.

Details

Roles are pruned once they go beyond 95 users (the hard limit is 100). It will then attempt to prune 10 random users who fulfill:

• not recently joined (7 days)
• no recent message in a help thread (90 days - this is also the lookback limit of our DB as of today)

Users are send a DM to inform them about what happened:

The routine also logs on INFO level who got pruned, so we can always double check in the logs. It will also tell us on WARN level if a role is, despite auto-pruning, approaching or even going beyond the limit. So that we can start to prune manually.

Latter warnings are also forwarded to the #mod_audit_log channel, so that we really do not miss it:

Other stuff

This PR also addresses two unrelated things:

• Added hints on how to change the logging level (also for bot only) in log4j2.xml
• Fixed a bug with routines starting too early (they must only be started once the system is ready, not directly during startup)
• Fixed a bug where BotCore#onReady is not always triggered because the core is added too late to JDA

[marko-radosavljevic]

Clean. ☺️

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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