Reorganize source files of collab::db

* Move all sea_orm tables into a 'tables' module
* Move TestDb into its own file
* Move id types into their own module

crates/collab/src/db/tables/channel_path.rs

@@ -0,0 +1,15 @@
+use crate::db::ChannelId;
+use sea_orm::entity::prelude::*;
+
+#[derive(Clone, Debug, Default, PartialEq, Eq, DeriveEntityModel)]
+#[sea_orm(table_name = "channel_paths")]
+pub struct Model {
+    #[sea_orm(primary_key)]
+    pub id_path: String,
+    pub channel_id: ChannelId,
+}
+
+impl ActiveModelBehavior for ActiveModel {}
+
+#[derive(Copy, Clone, Debug, EnumIter, DeriveRelation)]
+pub enum Relation {}