Wagtail <-> L10N workflow improvements

[stevejalim]

Description

We have a core workflow operational with Smartling thanks to https://github.com/mozilla/wagtail-localize-smartling but there are some additional things we need to do (mostly within Bedrock, not wagtail-localize-smartling) to improve the workflow.

Tasks

Beta Give feedback

1. URLs that use prefer_cms should merge active_locales data #15349
   Bug 🐛 +1
2. Support notifications or auto-publishing when a translation lands back #15347
   +0
3. Support Smartling's CAT tool wagtail-localize-smartling#34
   enhancement +1
   
4. Track the wagtail-localize issue with auto-translation of subtrees failing, to completion
5. Track the wagtail-localize issue about the translation UI misbehaving to completion