Add ability to update messages that have already been issued using PyBot

[raamink]

Implementation note:
We will need to keep track of the bot's message history.
There is one factor that complicates this: the bot's message history (message cache) clears itself after it is rebooted, meaning that in the event of a crash or code update, we lose the cache. In the past it was easy to add messages to the internal cache again as it was simply a list. However, in recent updates to the Discord API, this has been changed. Therefore keeping a separate message history and retrieving the actual messages using API calls will be a better approach.