feat(Settings): Add unset_setting and unset_settings fns to App

[brennie]

Closes #590.

[yo-bot]

Thanks for the pull request, and welcome! The team is excited to review your changes, and you should hear from @Vinatorul (or someone else) soon.

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[kbknapp]

Excellent! Thanks for knocking this out! @homu r+

[homu]

📌 Commit 8b06d5a has been approved by kbknapp

[homu]

⌛ Testing commit 8b06d5a with merge 16028ea...

[homu]

☀️ Test successful - status

[homu]

👀 Test was successful, but fast-forwarding failed: 422 Required status check "continuous-integration/appveyor/pr" is expected