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