On launch select the lock screen, the entry list or the welcome screen

[jhugman]

When I return to Lockbox, I want to know how to unlock my app if the prompt doesn't automatically appear.

Acceptance criteria

• Should provide clear call to action to unlock the app when the app is locked

Visual Design

InVision
Lock Screen: https://mozilla.invisionapp.com/share/3SN35UHWJDX
Unlock (Fingerprint): https://mozilla.invisionapp.com/share/3SN35UHWJDX#/310606417_Lock_Screen_-_Fingerprint

Zeplin
Lock Screen: https://zpl.io/25qK4En
Unlock (Fingerprint): https://zpl.io/bJq3DEJ

As a follow up to PR #133; this story shows the user the correct screen on launch, depending on the state of the app.

• If the app is not logged in, show the welcome screen.
• If the app is logged in, but unlocked, show the entry list.
• If the app is logged in, but locked, show the lock screen.

Related to #54
depends on #22