Configurable retention

[michielp1807]

TODO:

• make project settings set retention period
• make project creation choose a retention period
• validate on server in create/update that retention period is reasonable based on subscription
• add tests for retention period verification
• make remove_expired_message_data use the project specific limit instead of '30 days'
• what to do if an organization downgrades/ends their subscription? force lower retention period limit?
• update existing tests for project-specific retention period?

Closes #424

[codecov]

Codecov Report

❌ Patch coverage is 84.78261% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/models/projects.rs	33.33%	4 Missing ⚠️
src/models/organization.rs	85.00%	3 Missing ⚠️

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