Do not add Aliases for translated pages if they are not ticked for actual translation

[stevejalim]

By default when a user adds a new page in en-US, wagtail-localize adds an Alias page in each of the (10) configured languages that we currently support. However, the problem is that if you go to /fr/some-new-page/, the URL path stays the same, but displays the unlocalized content from /en-US/some-new-page.

This automatic creation of Aliases is done in order to preserve a synchronised page tree between locales, but it doesn't really suit how we want our page tree to be, and the Aliasing behaviour is not what we want - if there is no page specifically published for /fr/some-new-page/ we should 404, rather than show the content of the en-US version of that page.

As such, we need to find a way to disable the auto-generation of aliases in a clean, clear and documented way.

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Examples of other people with this issue:

• It should be possible to create a Page model not translatable wagtail/wagtail-localize#586

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Success criteria

• Page aliases are not automatically created for new pages
• Page aliases are not creatable at all via the UI

[stevejalim]

Note that completely disabling the Alias would also be a (somewhat sledgehammer) solution to mozilla/wagtail-localize-smartling#22, too

[stevejalim]

Well, that was a useful Zoom with @alexgibson - while helping him set up a local state to test #15066 we spotted that we could just not enable locale syncronisation for a Locale... and all the pain goes away.

Closing as invalid / StevePEBCAK