Maintenance: clear output adapter destinations

As part of the run-from-console "maintenance" workflow of the Overseer (see #20 (closed)), add support to clear configured OutputAdapter destinations:

• Console: no-op (no effect on a running Overseer)
• Discord: delete all messages from channel; must be batched / throttled to handle delete of thousands of messages while respecting rate-limiting clone channel, delete source channel, then rename clone (may require elevated bot permissions)
• File: (future) truncate the file
• Websocket (see #37): send a "clear" message, consumer can handle it if they choose