Simplify publish date checks
Simplify publish date checks

Fixes #814

- Changed to compare instances of `moment` using it's built in
comparison function instead of string compare with the date from the
database.
- Ensure compared date is parsed with the correct formats that we were
supporting before #1107.
- Moved date formats to variables to reduce chance for error in future.

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