Add extra check to bot startup to ensure all roles within `STATISTICS_ROLES` list exists

This check will need to be performed in both the `on_ready` event, and also within the future command that will allow changing of config variables (likely to be `/config set `), implemented within #221.

It has to be called from the event/command side rather than from within the config variable loading process, because the config loading is done before the connection to Discord is made (so the existent roles could not be retrieved).

Start by locating the Python bot's on_ready event and the configuration command planned in #221. Read how STATISTICS_ROLES is loaded and how Discord roles can be retrieved after connection. Done means both startup and configuration changes check that every role named in STATISTICS_ROLES exists.

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