Add subscribe button to global and sticky nav

[joshdarby]

Changes

This pull request makes the following changes:

• Adds a subscribe button to the global and sticky navs using the same approach we used for MWCIR Add grunt, less, etc. and add subscribe button to headers umbrella-mwcir#34

Desktop:

Sticky:

Mobile:

Why

For #66

Testing/Questions

Features that this PR affects:

• Global and sticky nav

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Is /subscribe/ the correct page to point the button to?

Steps to test this PR:

1. View the global and sticky navs on desktop and mobile and make sure it looks ok

[benlk]

Note that WPBuddy/largo#1848 will make this so much better in the future.

[benlk]

Code on this looks good, but I don't have an answer to this question:

Is /subscribe/ the correct page to point the button to?

@MirandaEcho, can you check with them to see if that's the correct URL?