Add session timeout support

Intercept all http(s) responses and store expiration time from headers
in local storage. Drive expiration timers in app container across all
tabs with browser storage events and accompanying react hooks
integration. Show a warning with logout countdown and continue button
when session is nearly expired.

awx/ui_next/docs/APP_ARCHITECTURE.md

@@ -0,0 +1,27 @@
+# Application Architecture
+
+## Local Storage Integration
+The `useStorage` hook integrates with the browser's localStorage api.
+It accepts a localStorage key as its only argument and returns a state
+variable and setter function for that state variable. The hook enables
+bidirectional data transfer between tabs via an event listener that
+is registered with the Web Storage api.
+
+
+![Sequence Diagram for useStorage](images/useStorage.png)
+
+The `useStorage` hook currently lives in the `AppContainer` component. It
+can be relocated to a more general location should and if the need
+ever arise
+
+## Session Expiration
+Session timeout state is communicated to the client in the HTTP(S)
+response headers. Every HTTP(S) response is intercepted to read the
+session expiration time before being passed into the rest of the
+application. A timeout date is computed from the intercepted HTTP(S)
+headers and is pushed into local storage, where it can be read using
+standard Web Storage apis or other utilities, such as `useStorage`.
+
+
+![Sequence Diagram for session expiration](images/sessionExpiration.png)
+