Admin API endpoint for creating/importing messages

As an additional note, one difficulty that may arise when implementing such a feature which I came across while doing manual message imports using the database is that currently channels don't seem to properly load messages that are in a bucket earlier than the bucket of the channel id. I'm not sure if there's a good way around this without somehow migrating a channel to a new ID if you try to add a message to a channel from before its creation date, or modifying the behaviour of how messages are loaded to not restrict the query to the bucket of the channel as the minimum bound (perhaps store a new field in the channel object in the db for what the minimum bucket in that channel is?).