Upgrade django to 2.1.15

requirements.txt

 # Core
 argon2-cffi==16.3.0		# Password hashing algorithm
 Babel==2.4
-Django==2.1.8			# 2019-05-18
+Django==2.1.15			# 2020-03-07
 feedparser==5.2.1		# Check: not updated since 2016
 psycopg2==2.7.3.2  		# PostgreSQL engine
 pytz==2017.2			# Timezone package