System message UI tweaks

- Add action-sheet-class option for action sheet directive
- Enable scrolling in system messages action sheet
- Partial fix for https://github.com/Rise-Vision/common-header/issues/396, related to https://github.com/Rise-Vision/common-header/pull/398
- Change "System Messages" title to "Notifications", fix for 
https://github.com/Rise-Vision/common-header/issues/395
- Sort system messages, fix for https://github.com/Rise-Vision/common-header/issues/393
@settinghead this is not important, but I think it's a good idea to move `var currentTime = new Date();` one line up so it's not re-created on very cycle inside the loop.