Add unique constraint on form + form definition

[sergei-maertens]

Closes #3527

• Added database migration to block duplicate form steps with a unique constraint (with ✨ [#3187] prevent selecting duplicate form definitions #3460 the UI was already updated)
• Added upgrade check script to detect duplicates, this will be backported to stable/2.3.x

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (87503a8) 95.92% compared to head (2d68aa3) 95.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3528   +/-   ##
=======================================
  Coverage   95.92%   95.92%           
=======================================
  Files         672      672           
  Lines       21532    21532           
  Branches     2481     2481           
=======================================
  Hits        20654    20654           
  Misses        609      609           
  Partials      269      269           

Files	Coverage Δ
src/openforms/forms/models/form_step.py	100.00% <ø> (ø)
src/openforms/upgrades/upgrade_paths.py	98.80% <ø> (ø)

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