Changes in Data-Review, settings.py and additions in requirements.txt files

**Added dependencies in Requirements.txt**
- Dependencies ```stemming```, ```django-registration-email``` are added to *requirements.txt*
- Search requirement ```numpy``` moved to *requirements.txt*  from *search_dependencies.txt*

**Discussion feature**
- In the view to create discussion *thread* ```@login_required``` added.
- This should resolve an error exist while creating discussion.

**Static list variables are added into ```settings.py``` file**
- This variables are named as ```GSTUDIO_RESOURCES_....  ```.
- The values for this lists are taken from spreadsheet given by *NCERT*.
- This lists are imported in *file.py* and rendered to data-review template.