Add subscribe button to global and sticky navs

[joshdarby]

Changes

This pull request makes the following changes:

• Adds a "Subscribe" button to the global nav and sticky nav next to the "Donate" button

(ignore the missing icon font in these screenshots) Desktop:

Sticky:

Mobile:

Why

For #83 / Giving Tuesday

Testing/Questions

Features that this PR affects:

• Global nav, sticky nav, mobile nav

Questions that need to be answered before merging:

• [ ] Is this PR targeting the correct branch in this repository?
• [ ] Are the colors ok?
• [ ] The donate button has an icon inside of it. Does the subscribe button need an icon as well?
• [ ] The donate button is hidden on the mobile menu. Should the subscribe button be hidden as well?

Steps to test this PR:

1. View the subscribe button on the global nav on desktop
2. View the subscribe button on the sticky nav on desktop
3. View the subscribe button on the mobile nav

[joshdarby]

Needs hover styles for the Subscribe button in the sticky nav, but otherwise good to ship.

You're right. https://github.com/INN/umbrella-borderzine/pull/84/commits/4ec5dd5e9d63d886dbf0eee654150a1b6caf7c4c switches the darken function to lighten and bumps it up to 15%