text: Title: Remove Null Option from status.Task.tags (M2M) and fix warning
  Body: **Describe the bug**
  On running the django app, the terminal shows the following warning
  ```
  WARNINGS:
  status.Task.tags: (fields.W340) null has no effect on ManyToManyField.
  ```
  
  **To Reproduce**
  1. Run the django server 
  
  **Expected behavior**
  The error should be avoided. If null option has no effect, it should be removed.
  
  **Screenshots**
  ![image](https://user-images.githubusercontent.com/21276922/51860350-6aab7e80-235f-11e9-97e8-d1360e4fa35c.png)