[User Onboarding] Add "Verify your phone number" modal after sign in

[garyhtou]

After signing into websites, sometimes they'll prompt you to...

• Double check your security settings
• Update your contact info (e.g. phone number, etc.)

Let's use this style of modal to prompt users to verify their phone number. We'll show this only after they sign in.

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I'll mention, I'm not completely in love with this UX because when you're signing into an app, you're doing so because you have a clear goal in mind. This style of UX adds a barrier and distraction to you completing that goal.

[garyhtou]

We currently have 8,132 users with unverified phone numbers who have a Stripe cardholder. We need to ensure this modal is implemently well before we merge.