Check if the correct user is authorized during OAuth

[pinarol]

Closes #370

Description

Calls /v3/me/associated-email to check if the correct user is authenticated right after oauth. Displays an error message otherwise:

Testing Steps

SwiftUI Demo app > Profile editor with oauth Login with email A first Then enter a different email in the SwiftUI app and enter the OAuth flow Tap "Approve" (since the browser remembers your previous login)

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar SwiftUI Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar SwiftUI Prototype Build
	Build Number	1314
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	1001bc328f178558917fa5fe5e1c3dc6cddb2aba
	App Center Build	Gravatar SDK Demo - SwiftUI #163

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar UIKit Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar UIKit Prototype Build
	Build Number	1314
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	1001bc328f178558917fa5fe5e1c3dc6cddb2aba
	App Center Build	Gravatar SDK Demo - UIKit #164

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.