Add User Notification Upon Approval

Add user notification upon approval/activation

- Uses same logic as registration email to determine from address
- Refactor of various registration templates for readability
- Refactor of various registration tempaltes for HTML multipart
fixing page permission cache identifier bug to correctly identify when to use permission collection id; making 5.5.2.1 upgrade to 5.6.1
Merge pull request #929 from mkly/feature_user_approved_notfication

Add User Notification Upon Approval
Merge branch 'master' of github.com:concrete5/concrete5