Enforce boolean defaults on Application Settings

What does this MR do?

Enforces defaults and disallows nulls on application_settings boolean columns.

Why was this MR needed?

New application settings have two "defaults":

• New install: When a record does not exist in application_settings, we use ApplicationSetting.defaults

• On upgrade: When a record exists, the value defaults to the column default

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2750 (closed)