WordPress share activity uses correct modal presentation style.

Show share activity via a popover on WPWebViewController on iPad.
Merge pull request #2156 from wordpress-mobile/issues/2153-share-activity-fix

WordPress share activity uses correct modal presentation style.
Updating localized strings.
Improves sanitizing featured image string.
Merge pull request #2161 from wordpress-mobile/issues/2157-featured-image-string-crash

Improves sanitizing featured image string.
Merge branch 'release/4.2' into develop
Exposes notificationsViewController property
Prevents instance duplication by state restoration
Merge branch 'issues/2178-fixes-state-restoration-duplicate-instances' into issues/1767-pushing-notification-details

Conflicts:
	WordPress/Classes/System/WordPressAppDelegate.h
	WordPress/Classes/System/WordPressAppDelegate.m
	WordPress/Classes/ViewRelated/Notifications/NotificationsViewController.m