Update django to 2.1.8 (#1717)
Update django to 2.1.8 (#1717)

This PR updates [Django](https://pypi.org/project/Django) from **2.1.7** to **2.1.8**.



<details>
  <summary>Changelog</summary>
  
  
   ### 2.1.8
   ```
   ==========================

*April 1, 2019*

Django 2.1.8 fixes a bug in 2.1.7.

Bugfixes
========

* Prevented admin inlines for a ``ManyToManyField``\&#39;s implicit through model
  from being editable if the user only has the view permission
  (:ticket:`30289`).


==========================
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/django
  - Changelog: https://pyup.io/changelogs/django/
  - Homepage: https://www.djangoproject.com/
</details>


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