Annotation Based Discussions

Added a feature to enable discussions based on some selected text.
- Currently, works for pages, files and groups.
- To enter annotation mode either click on the Annotate icon or select some text. 
- Select text to add annotations.
-Your selection shall be automatically extended to include entire words.
- To view already existing comments, click on the respective text (shown in different color).
- Annotations on overlapping text are allowed.  
- To exit the annotation mode, click on the Annotate icon.

To ensure proper working of the code, please run the following commands from the root level of your gstudio project (where manage.py file resides):
``` bower install```
```python manage.py add_fields_to_existing_documents```