Auto sync and machine translation

[th3hamm0r]

This only partially implements the proposed changes of #789 (comment) and should be seen as an early draft now open for discussion.

TBD:

• Publication + automatic machine translation: the attribute on the request looks a bit ugly, but it was the easiest and in my view the lightest approach (in contrast to sub-classing the EditView for Page).
• I've left out snippets for now, but I could mostly mirror the page logic, but listen for the wagtail.signals.published signal instead of the after_publish_page hook (actually this signal could be used for pages too 🤔 )
• I've skipped bulk sync for now, as it requires quite a lot of extra code and UI
• The go_live_at date isn't synced at the moment (which is understandable, as there are other use cases too), so I had to skip the publish+machine-translation for future dates. I've ignored this in the auto-sync on publish action, as it looked sensible to me to get the new content into the translations when a publication is planned.

Todo:

• Tests
• Docs

Relates to #789

[th3hamm0r]

With publish+auto-machine-translate we cannot really handle those components, I think. With bulk actions we may add those components to the confirmation view. That's why I have not extracted them into update_translations().

I also don't fully understand them. Are those components meant to be created once per translation request or once per translation source? I know, it's up to the dev, but it would be nice to get some infos on how this has been used so far.