Add message_history table

[emmiegit]

The messages table tracks the current state of all messages. However any information in edits is lost. We should add a message_history table with the following schema:

Column Name	Type	Other
message_id	BigInteger	Primary key, foreign key messages.message_id
date	DateTime	Primary key
content	UnicodeText
embeds	JSON

This way we preserve the existing messages table and also don't duplicate tons of information over several rows. For instance, the message_type or channel_id of a message won't change over edits, and so can be fetched by JOINing on messages.