[#2753] Make contact form configurable via CMS

[pi-sigma]

Taiga: https://taiga.maykinmedia.nl/project/open-inwoner/task/2753

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.89%. Comparing base (f0aedc1) to head (445977a).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1402   +/-   ##
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  Coverage    94.89%   94.89%           
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  Files         1046     1049    +3     
  Lines        38612    38655   +43     
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+ Hits         36640    36682   +42     
- Misses        1972     1973    +1     

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	94.89% <100.00%> (+<0.01%)	⬆️

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[swrichards]

Just a few small things: it's clear a few hoops had to be jumped through, but I don't have any better ideas, so this approach LGTM.

[pi-sigma]

@swrichards messages are not being cleared between subtests in test_submit_and_register_kvk_or_rsin_user_via_api. I've tried various things to no avail; somehow the combination of subtests and requests_mock prevents manual clearing of messages in the test. I am now clearing messages in set_result_message of the ContactFormView. This is not optimal, since the problem is with our test, not with the business logic (at least that's my suspicion, since the frontend shows only one message; on the other hand, the problem only occurred after introducing the CMS plugin). If you have ideas about this, let me know.

[swrichards]

@swrichards messages are not being cleared between subtests in test_submit_and_register_kvk_or_rsin_user_via_api. I've tried various things to no avail; somehow the combination of subtests and requests_mock prevents manual clearing of messages in the test. I am now clearing messages in set_result_message of the ContactFormView. This is not optimal, since the problem is with our test, not with the business logic (at least that's my suspicion, since the frontend shows only one message; on the other hand, the problem only occurred after introducing the CMS plugin). If you have ideas about this, let me know.

As discussed off-line: clearing the cookiejar on the app object appears to solve this issue.

[alextreme]

This PR doesn't tackle what I also wanted, namely a general plugin for content so that we can phase out flatpages in favor of using CMS Pages. But it was out-of-scope for the user story