Standardising notification types
Standardising notification types

 - CSS classes directly correspond to notification 'types'
 - Error, warn and info are reasonably standard terms for descending priority 'log levels', using these to denote red, orange and blue notifications.

kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,JohnONolan/Ghost-Admin,acburdine/Ghost-Admin,dbalders/Ghost-Admin,JohnONolan/Ghost-Admin,TryGhost/Ghost-Admin,airycanon/Ghost-Admin,kevinansfield/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,dbalders/Ghost-Admin