Add always_cohort_inline_discussions in course settings

For a cohorted course, we needed a way to set inline discussions non-cohorted by default and select manually the discussions that need to be cohorted.  2 course settings added:

* always_cohort_inline_discussions: Set it to False to get the inline discussions NON-cohorted by default for a cohorted course. Then, you can add the inline discussions id in the cohorted_discussions array:
``` 
"cohorted_discussions": [
        "c2293fa2538a41eca7224b8a07c3d09d"
    ],
```
The implementation is simple and doesn't affect much code. 

cc @antoviaque
Hi, @Kelketek. It is used by video module. Some deprecated fields need to be clearable without possibility to change the value. Appropriate code and more information you can find here:
https://github.com/edx/edx-platform/blob/master/common/lib/xmodule/xmodule/video_module/video_module.py#L292