Twenty Fifteen: Skip to content font and styling

[afercia]

Very subtle thing:

• "Open Sans" is not available when not logged-in, unless it's installed in your system fonts, so it fallbacks to sans-serif which depends on platforms and devices, should use "Noto Sans"
• when logged in, should use "Open Sans" and have the same style as "Skip to toolbar" and "Log out" for consistency
• should use a selector to target just the skip-link

"Noto Sans" bold is "bolder" than "Open Sans" bold but I guess we can live with that.
For testing, be sure "Open Sans" is *not* installed on your system.

See screenshots, where I've moved and made the 3 links temporary visible to better compare them.

logged in and logged out "before" on Windows, fallbacks to Arial:

logged in and logged out "after":

[iamtakashi]

Cleaned-up patch.

[iandstewart]

@iamtakashi can you add a visual record? Screenshots! :)

[iandstewart]

In 29976:

Twenty Fifteen: updated styles for skip link.

Props afercia and iamtakashi, fixes #30043.