Groups' dropdown list updated - Restricts group and/or user group(s) listing to limited values


**Functionalities implemented:**

1) For logged-in users, the dropdown lists only those group(s) to which user is subscribed to and also it's user-group at the bottom of the list; excluding the currently selected group from the list

2) For anonymous users (without logged-in), the dropdown lists only public group(s); excluding the currently selected group from the list

   - Files modified:
   
   1) templatetags/ndf_tags.py:
      - ```get_user_group()``` modified (For logged-in users)
      - ```get_existing_groups_excluded()``` modified (For Anonymous users)
      
   2) templates/ndf/base.html
      - Both tags syntax modified
      - li element added for listing ```See more groups...``` option 

NOTE: In both cases, three new things are also applied to the dropdown, which are as follows:
1) "See more groups..." option is provided at the bottom of the list which takes user to group's list-view 
2) Because of the above option, only top 10 groups are listed which are sorted based on last_update field (in descending order)
3) Group names that is either highlighted as selected or in the list, is truncated to 30 characters.