Prevent exception when doing PUT and ZaakTypenRelatie already exists

[shea-maykinmedia]

Fixes #851

Changes

When doing a PUT on the zaaktypen endpoint we want to delete ZaakTypenRelatie that were previously created so that they can be recreated in this PUT request.

[codecov-io]

Codecov Report

Merging #875 (e8d284d) into master (a60d2d4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #875   +/-   ##
=======================================
  Coverage   96.13%   96.13%           
=======================================
  Files         513      513           
  Lines       30817    30843   +26     
  Branches     1517     1518    +1     
=======================================
+ Hits        29625    29651   +26     
  Misses        938      938           
  Partials      254      254           

Impacted Files	Coverage Δ
...nzaak/components/catalogi/api/viewsets/zaaktype.py	100.00% <100.00%> (ø)
...penzaak/components/catalogi/tests/test_zaaktype.py	100.00% <100.00%> (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 a60d2d4...e8d284d. Read the comment docs.

[sergei-maertens]

Can you add a test where two existing relations are replaced by just a single one as well?