rewrites default private.hbs for new zelda styles
rewrites default private.hbs for new zelda styles

closes #5073
- changes format to match new zelda layout
- modifies the input_password helper to allow customization of class & placeholder

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