Refactor ContentLog class. Use FloatField instead of IntegerField for progress field.

@aronasorman.

Branch:  develop
Issue: https://github.com/learningequality/ka-lite/issues/2767
Related PR: https://github.com/learningequality/ka-lite/pull/2790

I changed `IntegerField` to `FloatField` for percentage use. 

Attached screen shoot:
![Sample](https://raw.githubusercontent.com/djallado/Files/master/floatfield.png)