Headers: Use native sticky local navigation bar & site breadcrumbs

[ryelle]

This will be necessary if WordPress/wporg-mu-plugins#466 merges, because the sticky behavior will change. This should make it easier to implement sticky bars on child themes, but does require updating the current sites that are using the older is-sticky class.

Other changes I made:

• Removing the padding on local-navigation-bar, this is set correctly by default now
• Move the border from local-navigation-bar to the global header, this looks better when the page scrolls
• Since the local-navigation-bar was moved out of the group, I moved the style attribute to local-navigation-bar directly

Screenshots

This fixes an issue with the homepage scroll, and makes the breadcrumbs sticky.

Screen	Before	After
Home
Home scrolled
Article
Article scrolled

How to test the changes in this Pull Request:

1. Apply the wporg-mu-plugins PR
2. View any pages — the global header is no longer sticky, local nav is, and breadcrumbs (when they appear) are.

[adamwoodnz]

2. View any pages — the global header is no longer sticky, local nav is, and breadcrumbs (when they appear) are.

@fcoveram I believe the breadcrumbs should not be sticky, correct?

Sorry to keep repeating this question! Just not sure if we're going with that for the whole network. I think this is the outlier now, all others do not have sticky breadcrumbs :)

[adamwoodnz]

LGTM, just the one existing question on the sticky breadcrumbs.

[fcoveram]

No worries @adamwoodnz for asking me again 😃 And yes, no sticky breadcrumb.