Promote FxA/Sync sign in when user is not logged in

[sandysage]

When I'm not signed in with a FxA to Sync (specifically Sync for logins), I need to first know how to do that.

Acceptance criteria

• Sign in to FxA and Sync should be the primary call to action if someone is not already
• Should promote the sign in from the top navigation

To consider: do we want to change the promotion at the bottom of the screen when not signed in?

Visuals

related #78
child of #38

[sandysage]

IRL discussion with Ryan, we do need to do a specific 'promotion' for signing into FxA/Sync. Leaving the flow as an open question (where we display the promo and how).

@changecourse is aware of this open request. It's my understanding he will not have an opportunity to provide designs here until the end of March given other commitments on Monitor.

[sandysage]

I propose we move forward with changing the promotion at the bottom of the screen to promote "sign in to get access across devices" when not signed in.

@changecourse can you provide assets for that promotion?

[changecourse]

@sandysage do we need more than just a copy change for header/subheader and CTA for the unauthenticated state? See attached mock-up. (we may need some copy love before finalizing)