Add possibility to dismiss admin messages

What's Changed

• change editor hook (WP-886) by @vsolovei-smartling in #528
• add import flow (WP-826) by @vsolovei-smartling in #526
• add possibility to dismiss admin messages (WP-844) by @vsolovei-smartling in #529
• fix target content not created (WP-889) by @vsolovei-smartling in #530
• properly display wait state on widget button (WP-887) by @vsolovei-smartling in #531

Full Changelog: 3.9.0...3.9.1