Rewrite pieces of code deprecated by Django 1.7

The mimtype argument of HttpResponse, SimpleTemplateResponse,
TemplateResponse, and render_to_response was removed; one should
use content_type instead

Change-Id: I62214c37264845dbabf006a32616e894c4ac9e3a
Closes-bug: #1266676
Partially-impelements: blueprint django-1point6
Merge "Rewrite pieces of code deprecated by Django 1.7"