Get the session token for the user's account, if one is available.

💾 This method alters the persisted account state.

Applications that function as a web browser may need to hold on to a session token on behalf of Firefox Accounts web content. This method exists so that they can retrieve it an pass it back to said web content when required.

Notes

- Please do not attempt to use the resulting token to directly make calls to the
  Firefox Accounts servers! All account management functionality should be performed
  in web content.
- A session token is only available to applications that have requested the
  `https:///identity.mozilla.com/tokens/session` scope.