Feature/2767 add timestamp fields to content log model

@aronasorman.

Branch: develop
Issue: https://github.com/learningequality/ka-lite/issues/2767

We add `start_timestamp` and `progress_timestamp` in `ContentLog` and add method save to save `progress_timestamp`.

We also try to perform this:

Setting up `ContentLog pdf`:

1. Create a directory like:
	/Users/Home/Data
2. Add pdf file on that directory
3. run this command: "manage.py contentload -i <directory> -c <channel name>" like: 
	python manage.py contentload -i /Users/Home/Data -c chan1
4. add `CHANNEL = "<channel name>" to local_settings.py`

ContentLogs work fine when student view pdf.

We have a recommendation on ContentLog Field below:
1. Change `progress = models.IntegerField(blank=True, null=True)` to `progress = models.FloatField(blank=True, null=True)`.

The purpose is to get percentage instead of whole number 1 return by `IntegerField` 



And then we are also testing the progress_timestamp value right?