Add newsletter subscribe button to the header and sticky nav

[joshdarby]

Changes

This pull request makes the following changes:

• Adds a Subscribe button to the header and sticky nav

Desktop:

Sticky:

Mobile:

Why

For #7

Testing/Questions

Features that this PR affects:

• Global nav and sticky nav

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?
• [ ] Is the blue color ok?

Steps to test this PR:

1. View the global nav, sticky nav, and mobile nav and verify they all look ok.

[benlk]

How do we want to handle the narrowest screens?

[joshdarby]

@benlk I think it's fine like that. Even at 200px width it still doesn't look horrible. Plus, if someone is using a screen at 200px I think they'll see more issues than just the subscribe button

[benlk]

There are probably still folks using 320px iPhones, though.

[joshdarby]

Right, but it still looks ok at that size.

[joshdarby]

@benlk can this be merged or do you have more concerns with it?

[benlk]

If you're okay with it, then sure. I don't like how the sticky nav doubles in height, but we don't have a good fix for that.