upgrade Django version to the most recent LTS version

At the moment we are using 1.8.17 (LTS) which has not been supported since April 1, 2018 (last compatible version is 1.8.19).

The last Python 2 compatible version is 1.11 (end of support April 2020).

Current LTS version is 2.2, Python 3 only (see #1 (closed)), supported until 2022.