Add option to translate a Page recursively

[jkevingutierrez]

Found a bug, please fill out the sections below

Issue summary

Having WAGTAILTRANS_SYNC_TREE = True syncs the whole tree of pages, but having WAGTAILTRANS_SYNC_TREE = False doesn't give an option to copy the whole tree of the page when someone wants to translate the page.

It could be useful to have a boolean option in the "Translate" page (/admin/translate/<PAGE_ID>/add/<LOCALE_CODE>) to allow the copy of all the children of the page to be translated.

This could be useful if you want to translate a page, and also want the children of that page to be copied to the translated page