Integrate native Gravatar Quick Editor to change avatar.

[etoledom]

This PR is a demonstration of integrating the native Gravatar Quick Editor, replacing the web-view interface.

The secrets can be shared internally.

To test

• Add the Gravatar secrets to the (temporal) GravatarSecrets struct shared internally.
• Go to Profile -> Account -> Change Avatar
• Authenticate your Gravatar account.
• Test the integration of Gravatar Quick Editor with Pocket Casts.

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

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[dangermattic]

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