Allow Users to Destroy Account and Add Batch Action for Emailing

**Makes User Accounts Destroyable**  
This pull request gives users the ability to destroy their account on the server and all the posts associated with it.

**Added Batch Action for Sending update email to users**  
Go to the admin interface for users, select a few user accounts and select the batch action from the upper left. It will send the email written at `app/views/notifier/update_invited_user.html.erb` (and the text format version as well).

These two updates are conceptually linked because we can't send many emails without "unsubscribe" functionality. Since we are not marketing to users, a "destroy account" button is more relevant than an unsubscribe button. So users can destroy their account to unsubscribe.