Add unique constraint on group name

[flomonster]

Without this constraint some commands does not work properly like editoast group include.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.78%. Comparing base (28c965c) to head (922d8df).
Report is 1 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #9827      +/-   ##
==========================================
- Coverage   37.79%   37.78%   -0.02%     
==========================================
  Files         994      994              
  Lines       91192    91192              
  Branches     1176     1176              
==========================================
- Hits        34468    34453      -15     
- Misses      56270    56285      +15     
  Partials      454      454              

Flag	Coverage Δ
editoast	72.95% <ø> (-0.06%)	⬇️
front	20.09% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	86.74% <ø> (ø)

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[leovalais]

Looks fine to me, but do we really want groups to have a unique name? I hardly believe that a purely technical constraint like the CLI API is a good enough reason to ask users to never duplicate group names.

This also raises some UX questions: I (a lambda user) create a group with the name "My Group", which gets rejected. How do we display the error? Do we just show "A group named My Group already exists. Please choose another name."? (We can't show the group owner.) This is rather classic for user IDs and we can easily understand why, but for group names, as a user, I can hardly see the rationale behind that.

Wdyt?