#1220 Store no passwords for WordPress.com

Let's stop storing passwords for WordPress.com accounts.

Ideally, we need a patch server side to be able to show authenticated web views with just an OAuth2 token. While that is being created, I have an alternative approach for the two blockers:

- Open web stats on Safari and let the user authenticate there if necessary
- Show a simple preview for drafts in WordPress.com

cc @sendhil 

Fixes #1220 
You could probably purge WPChromelessWebViewController and WPWebView along with StatsWebViewController since nothing else would be using them.