Fix external content source blog when cron runs from non-source blog

What's Changed

• switch to source blog when working with external content (WP-877) by @vsolovei-smartling in #521
• use Smartling placeholders for shortcode detection (WP-879) by @vsolovei-smartling in #520
• ignore site for submissions with same batchUid (WP-874) by @vsolovei-smartling in #518

Full Changelog: 3.8.11...3.8.12