Edit canonical page

[jkevingutierrez]

Found a bug, please fill out the sections below

Issue summary

The canonical page of translated pages can't be edited using the Wagtail interface

A use case can be: The user creates the translated version before the original version.

For example, having the locales en (default) and es. You are able to add children pages directly to the es folder. The children will also have the language as es, but it won't have a canonical page (which is ok at this point, cause it doesn't exist). But later, you want to create the en version of the page.

If you create the en page, you are not able to link the es page with the en page as the field Canonical page is not editable

How to reproduce?

Having

/   (TranslatableSiteRootPage)
    en/        (TransHomePage)
        english-page-1/    (TransLandingPage)

    es/        (TransHomePage)
        spanish-page-1/     (TransLandingPage)

Create a child page directly in es (spanish-page-2).
Then create the english version in en (english-page-2)

The field Canonical page: under settings is not editable, so you can't link english-page-2 and spanish-page-2 using the Wagtail interface

Technical details

Python 3.8
Django==3.0.9
wagtail==2.10
wagtailtrans==2.1

[jjanssen]

When developing wagtailtrans a couple of years back our main assumption for the synchronised tree was that the default language would be most leading and therefore creating a page in the default language would create a draft in other languages. But perhaps it makes sense if this would also be the case non-default languages creating a draft in different languages.

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For reference, related to #172 but with a bit of different use-case.