check for needed configuration settings at the start of bot components

[trz42]

Changes to configuration settings in https://github.com/EESSI/eessi-bot-software-layer/pull/263 (e.g., renaming tarball_prefix to artefact_prefix) may lead to bot components crashing when those settings are accessed, but weren't changed in the app.cfg.

We should ensure that all needed settings are present for the event handler and the job manager by using something similar to what is done for the job manager in eessi_bot_job_manager.py:

• defining needed settings by https://github.com/EESSI/eessi-bot-software-layer/blob/2d15e7713d1d0f1835b2329d3a6200b810b7b0fc/eessi_bot_job_manager.py#L70-L77
• verifying the config settings are present by https://github.com/EESSI/eessi-bot-software-layer/blob/2d15e7713d1d0f1835b2329d3a6200b810b7b0fc/eessi_bot_job_manager.py#L611

This should be done before making a new release or we might miss to update the bot configs.