Add ability to support custom styles

Not using it yet, just thinking about usability and design a bit.

src/App.tsx

@@ -1,5 +1,5 @@
 import "./App.css";
-import "bootstrap/dist/css/bootstrap.min.css";
+import "./style.scss";
 
 import Client, { IAuth } from "./client/Client";
 import { AccountIdMap, IAccount, IMailbox } from "./client/types";

src/style.scss

@@ -0,0 +1 @@
+@import '~bootstrap/scss/bootstrap';