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.
Site Verification tools: modify output to add HTML comment

See #355
Site Verification: add conditional brackers, fix indentation
Use new get_active_plugins function everywhere

See e86d0ab4aeadde96cc3c6026867b8b1252bbdf2d
Prevent warnings when no plugins are active

Closes #353
Remove extra check for active plugins
Merge pull request #388 from Automattic/ui-tweaks-dave

Tagline and Messages formatting
Merge pull request #382 from jeherve/353-github

Use new get_active_plugins everywhere, and prevent warnings when no plugins are active
Merge pull request #357 from jeherve/ver-tools-comment

Site Verification tools: modify output to add an HTML comment
Multisite: Partially revert 5ee811895d1138aebc046e8001b5b296d235807e

See #382: that class is meant to check non network active plugins.
Merge pull request #134 from bbrooks/sso-singleton

Update Jetpack_SSO to be a proper singleton
Push Notifications: Remove module extra info.

Resolves #175
Moved site disconnect links to footer

Moved disconnect links from header to footer, as the newly re-added
help tab covers the disconnect link in the header.