Add scheduled email project reports subscription

UI

• Add the option to subscribe to reports on the project/app overview
• Make it possible to unsubscribe
• Add report frequency setting at global & project-level

Backend

• Update endpoints to have a `branchKey` optional parameter - when no branchKey is provided, we'll default to the main branch
  • api/project_reports/subscribe
  • api/project_reports/unsubscribe
  • api/project_reports/status →  canSubscribe, frequency, subscribed
• New frequency setting: sonar.governance.report.project.frequency
• Trigger sending emails to subscribed users respecting the frequency setting