Allow users to enable FaceID in settings after denying the app permission to use FaceID

[sashei]

Acceptance Criteria

When the user enables FaceID via the onboarding flow (or for the first time via the Settings page), they are prompted with a FaceID permission dialog (need text), and authenticate once with FaceID to enable it for the app.

Even if FaceID has not been enabled / allowed for the application yet, FaceID shows up in the Settings list as an option.

Permission Prompt Copy
Title: Do you want to allow 
“Lockbox” to use Face ID?
Subtitle: Enabling Face ID allows for quick and
secure access to your account.

Visual Design

InVision
Settings w/ Face ID (disabled): https://mozilla.invisionapp.com/share/7JEUG47ZU#/292189929_Settings_-_Face_ID_Auth
Permisson Prompt: https://mozilla.invisionapp.com/d/main/#/console/12774944/292189928/preview
Auth: https://mozilla.invisionapp.com/share/7JEUG47ZU#/292189929_Settings_-_Face_ID_Auth

Zeplin
Settings w/ Face ID (disabled): https://zpl.io/a7yE6WK
Permission Prompt: https://zpl.io/aXz4Ewg
Auth: https://zpl.io/V0E06qE

[sashei]

hey @sandysage , I think there's a product question here about whether or not this should be a P1. I've updated the story title & acceptance criteria; let me know if you have any questions.

[sandysage]

While being able to enable faceid to unlock from settings, is an ideal. We're expecting users to set that up primarily in the onboarding flow.

I can't advocate for greater support around faceid, for MVP, when it's currently only available to 3.3% of iPhone users: https://david-smith.org/iosversionstats/ given everything else in the queue.

We can revisit this if we discover a larger proportion of users are using iPhone X, the onboarding faceid setup is failing, and this is impacting retention.

[sandysage]

Another implication of not having the ability to enable FaceID in settings, is that we'll need to remove that toggle from the settings screen. And if it's turned on, in onboarding, it's on forever. Am I right @sashei?

[sashei]

If the user grants the app permission to use FaceID in the onboarding flow right now, the setting will be available even if they fail FaceID auth. However, if the user denies the app permission to use FaceID in the onboarding flow, the setting will never appear.

[sandysage]

That makes sense. Thanks @sashei. I've removed the MVP label and marked this as a P3.