Implement starboard extension

[netux]

This PR is based off #22. Merge that first.

Adds a starboard extension which allows users to star (give ⭐ reaction) to their favorite messages and get them to the starboard channel. This is useful for keeping track of the best messages sent on the server, or to simply take things out of context.

Server owners can configure the starboard channel and starboard threshold through the !config command:

Breaking Changes

This PR adds two new columns to the config table.

ALTER TABLE config ADD COLUMN starboard_channel VARCHAR(18);
ALTER TABLE config ADD COLUMN starboard_threshold SMALLINT;