Add migration and schema for accounts

Add account name availability endpoint

priv/repo/migrations/20210622142112_add_accounts_table.exs

@@ -0,0 +1,11 @@
+defmodule MatrixServer.Repo.Migrations.AddAccountsTable do
+  use Ecto.Migration
+
+  def change do
+    create table(:accounts, primary_key: false) do
+      add :localpart, :string, primary_key: true, null: false
+      add :password_hash, :string, size: 60, null: false
+      timestamps(updated_at: false)
+    end
+  end
+end