Allow example app and tests to be run with Django 2.2

[sliverc]

Description of the Change

• admin app would need several middlewares and other apps configured, hence removed for example app
• Remove invalid authors references in blogentry.json

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #642 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #642   +/-   ##
=======================================
  Coverage   96.16%   96.16%           
=======================================
  Files          56       56           
  Lines        2866     2866           
=======================================
  Hits         2756     2756           
  Misses        110      110

Impacted Files	Coverage Δ
example/settings/dev.py	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b207f99...5291c2f. Read the comment docs.

[n2ygk]

Interesting that only Django 2.2 surfaced this.... Perhaps pre-2.2 didn't default a foreign-key integrity check.

LGTM.