feat: add listing logger channels in a guild

This commit also allows in the database to hold more than one channel
per guild and introduces clippy linting.

Closes #2

BREAKING CHANGES: The database schema changed from its root. All
databases should be dropped and recreated before running this new
version.

migrations/20231122220824_guild_log_channels.sql

@@ -7,7 +7,8 @@
 -- them from the database. This operation is noop in Rust and should
 -- therefore not cost a single CPU cycle.
 CREATE TABLE IF NOT EXISTS guild_log_channels (
-       guild_id INTEGER PRIMARY KEY,
+       guild_id INTEGER NOT NULL,
        channel_id INTEGER NOT NULL,
        UNIQUE(guild_id, channel_id)
 );
+CREATE INDEX IF NOT EXISTS guild_log_channels_guild_id ON guild_log_channels(guild_id);