feat: allow network admin to control ephemerality setting

Resolves #92

go/migrations/000011_message_retention.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE networks
+  DROP COLUMN IF EXISTS message_retention_hours;

go/migrations/000011_message_retention.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE networks
+  ADD COLUMN message_retention_hours INTEGER NOT NULL DEFAULT 24;