Implement client joined rooms endpoint

Track which rooms a local account has joined
Add some documentation to modules

priv/repo/migrations/20210823083642_create_joined_rooms_table.exs

@@ -0,0 +1,16 @@
+defmodule MatrixServer.Repo.Migrations.CreateJoinedRoomsTable do
+  use Ecto.Migration
+
+  def change do
+    create table(:joined_rooms, primary_key: false) do
+      add :localpart,
+          references(:accounts, column: :localpart, type: :string),
+          primary_key: true,
+          null: false
+
+      add :room_id, references(:rooms, type: :string),
+        primary_key: true,
+        null: false
+    end
+  end
+end