feat: add uniqueness field validation for span questions

[frascuchon]

Description

This PR adds a server validation to avoid creating several span questions using the same field.

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the type of change)

• New feature (non-breaking change which adds functionality)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

(Please describe the tests that you ran to verify your changes. And ideally, reference tests)

• Test A
• Test B

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.33%. Comparing base (5cd1019) to head (dfe0c34).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           feat/support-for-span-questions      #60   +/-   ##
================================================================
  Coverage                            90.33%   90.33%           
================================================================
  Files                                  189      189           
  Lines                                 9289     9292    +3     
================================================================
+ Hits                                  8391     8394    +3     
  Misses                                 898      898           

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[jfcalvo]

Looks good to me. I added some comments in the case that you want to improve it a little bit.

[jfcalvo]

I already added in one commit a property to Question database model to check if if span or not. But after some changes I removed it because I was not using it.

Take a look to the commit here: 799743b#diff-b7efe64daa88a5d7c98088e2a66d38739605226e60d4ed967712ca2790c54931

If you want to apply that change then you can do:

Suggested change

	if question.type == QuestionType.span and field == question.parsed_settings.field:
	if question.is_span and field == question.parsed_settings.field:

[frascuchon]

I prefer to keep it as is. We already have other places using this kind of condition with settings directly and it can be confusing seeing is_span or settings.type == QuestionType.span depending on the context.

We can review it later when using question validators. However, migrating the project to Python 3.10 and using the pattern-matching switch would be a better option.