Refactor automod to use message filters #542

Currently, message filters perform a check that ensures the filters are not executed if certain automod conditions match (e.g. suspicious link).
When automod is made a filter that runs before all other filters, this would no longer be necessary.

The AutoMod class should be refactored to be a MessageFilter that runs before other filters (by overriding getOrder from #541 and returning HIGHEST_PRECEDENCE) and the check for automod in MessageFilterHandler#shouldRunFilters should be removed.

If the automod logic deleted a message, it should return MessageModificationStatus.STOP_PROCESSING and MessageModificationStatus.NOT_MODIFIED otherwise.
Even when it is made a filter, automod should still run on message edits so the edit listener should remain.