feat: Allow to configure notifications_sending_frequency

[Quentinchampenois]

🎩 Description

Please describe your pull request.

Allow to configure notifications_sending_frequency with ENV var NOTIFICATIONS_SENDING_FREQUENCY

📌 Related Issues

Link your PR to an issue

• Related to feat: allow to configure notifications_sending_frequency decidim-module-extra_user_fields#1
• Notion card

Testing

Describe the best way to test or validate your PR.

Example:

• Run server with env var : NOTIFICATIONS_SENDING_FREQUENCY=none bundle exec rails s
• Create registration
• Login and check "My account"

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

All modified and coverable lines are covered by tests ✅

Project coverage is 86.12%. Comparing base (a80cb3c) to head (b262ebd).

Additional details and impacted files

@@             Coverage Diff             @@
##           rc/0.27     #183      +/-   ##
===========================================
- Coverage    87.57%   86.12%   -1.46%     
===========================================
  Files           68       68              
  Lines         1650     1650              
===========================================
- Hits          1445     1421      -24     
- Misses         205      229      +24     

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