(guild admin): Add greeting messages.

New `SettingType` `Message` for general message settings.
New setting `greeting_message`.
New setting `greeting_channel`.
New setting `starting_funds`.
New setting `returning_message`.
Add a greeting message hook.
Add initial funds on lion creation.
Data migration v3 -> v4.

data/migration/v3-v4/migration.sql

@@ -0,0 +1,7 @@
+ALTER TABLE guild_config
+  ADD COLUMN greeting_channel BIGINT,
+  ADD COLUMN greeting_message TEXT,
+  ADD COLUMN returning_message TEXT,
+  ADD COLUMN starting_funds INTEGER;
+
+INSERT INTO VersionHistory (version, author) VALUES (4, 'v3-v4 Migration');