User privacy controls

Back end:
* Separate `PrivacySettingsForm` from `ProfileSettingsForm`
* User settings page: stay on page after save or cancel (per JeffF)
* Postback to user profile updates privacy settings

Front end:
* Add bootstrap modal to show privacy settings on profile page
* Module `privacySettings.js` controls toggling and display of privacy settings
* Used in both user profile and settings pages, each with their own save and cancel buttons