Move initial custom form creation to server

[progmatic-99]

Fixes #7071

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

Merging #8205 (fd8a992) into development (d8e2054) will increase coverage by 0.03%.
The diff coverage is 93.54%.

@@               Coverage Diff               @@
##           development    #8205      +/-   ##
===============================================
+ Coverage        65.74%   65.77%   +0.03%     
===============================================
  Files              305      305              
  Lines            15808    15828      +20     
===============================================
+ Hits             10393    10411      +18     
- Misses            5415     5417       +2     

Impacted Files	Coverage Δ
app/models/custom_form.py	98.18% <ø> (ø)
app/api/events.py	52.86% <33.33%> (-0.34%)	⬇️
app/api/helpers/events.py	100.00% <100.00%> (ø)

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[maze-runnar]

@progmatic-99 please complete (frontend changes too) and then test it by yourself end to end.

[mariobehling]

Please add the link to the frontend PR here as well. Thanks.