[2.x] Change OAuth Flow from Popup to Redirect #4428

Current Behavior

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Steps to Reproduce

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Expected Behavior

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Screenshots

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Environment

• Flarum version: x.y.z
• Website URL: http://example.com
• Webserver: [e.g. apache, nginx]
• Hosting environment: [e.g. shared, vps]
• PHP version: x.y.z
• Browser: [e.g. chrome 67, safari 11]

Output of php flarum info

Output of "php flarum info", run this in terminal in your Flarum directory.

Possible Solution

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Additional Context

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