Feat: Add extra user fields gem

[paulinebessoles]

🎩 Description

This PR aims to add the extra user fields gem to the decidim-app 0.27

Testing

Example:

• Log in as admin
• Access Backoffice
• Go to Settings
• See a dedicated tab for extra user fields

Tasks

• Add specs
• Add note about overrides in OVERLOADS.md
• In case of new dependencies or version bump, update related documentation

📷 Screenshots

Please add screenshots of the changes you're proposing if related to the UI

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -21.10% ⚠️

Comparison is base (6be0191) 77.06% compared to head (561cc1e) 55.96%.

Additional details and impacted files

@@              Coverage Diff               @@
##           bump_0.27     #425       +/-   ##
==============================================
- Coverage      77.06%   55.96%   -21.10%     
==============================================
  Files             60       54        -6     
  Lines           1574     1517       -57     
==============================================
- Hits            1213      849      -364     
- Misses           361      668      +307     

see 23 files with indirect coverage changes

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[armandfardeau]

Could you rebase or merge #366 ?