Update Jetpack_SSO to be a proper singleton

Remove usage of ‘new Jetpack_SSO’ in favor of Jetpack_SSO::get_instance
to emphasize that Jetpack_SSO is a singleton. Made the constructor
private.
Infinite: Only load new pages when within two screens of the bottom.

The current infinite scroll code loads a new page when the browser is
within 25% of the bottom of the page. On very long IS pages, this leads
to loading an increasing number of unnecessary pages, as the bottom 25%
of the page increases in size as the user loads more and more pages.

This patch makes IS less aggressive in loading new pages. Two screens was
chosen as a starting point and is obviously open to tweaking.
Site Verification tools: modify output to add HTML comment

See #355
Site Verification: add conditional brackers, fix indentation
Site Verification Tools: automatically activate the menu

It doesn't add anything to the site unless you fill in the options
so it could be auto-activated imo.
Open Graph: use the author's Website as og:url on author page

If it is defined in Profile
Add a hashtag to avoid w3c validation errors. Fixes https://plugins.trac.wordpress.org/ticket/2046
New Admin UI: translate text in footer
Admin footer: update the localization of the Automattic tagline

Also fix apostrophes in strings
Merge pull request #377 from jeherve/admin-ui-i18n

New Admin UI: a bit of i18n love in the footer
Use a custom cap for the general Jetpack Admin Page.

This will make it easier for folks to restrict the admin page if they
so choose.
Re-enable Admin Help.
Simplify our call to admin_page_load
Couple of minor CSS and copy tweaks

- Changed main CTA button to “Connect to Get Started”
- Copied connect button to bottom of homepage
- Changed jp-content z-index to 1 to fix modules from going over
navigation
- Removed GA code
Merge pull request #378 from Automattic/ui-tweaks-dave

Couple of minor CSS and copy tweaks
Help tab & section CSS tweaks

- Removed light gray border from help tab
- Added 20px green border to sides of help section
Merge pull request #379 from Automattic/ui-tweaks-dave

Help tab & section CSS tweaks
Redirect folks to the configure page.

This is temporary until it can be retooled to display the form in the
modal.
Playing with alert message location & styles

- Playing with new alert message design
- Fixed right border of search box
Merge pull request #380 from Automattic/ui-tweaks-dave

Playing with alert message location & styles