Add navigation bar to the avatar picker

[pinarol]

Closes #343

Description

Adds navigation bar. I used NavigationView for now since it is supported in iOS 15. NavigationStack is supported only for iOS 16+.

• Adds "Done" button to close the picker
• Gravatar icon opens Safari with profile url

Updates the "Discover Your Gravatar Card" to "View profile" based on designs. Ties the action to open the profile in Safari.

Refactors some of the paddings based on the designs.

Testing Steps

• Play around with the SwiftUI demo app.

[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	1118
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	4a4dd0b8d4fefcdd2b31810b8c5da850627895ce
	App Center Build	Gravatar SDK Demo - UIKit #54

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 SwiftUI Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar SwiftUI Prototype Build
	Build Number	1118
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	4a4dd0b8d4fefcdd2b31810b8c5da850627895ce
	App Center Build	Gravatar SDK Demo - SwiftUI #53

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

[pinarol]

I liked "fullScreenCover" better. 👍 For now it seems like the best we can get out of SwiftUI about this.