Enable a different og:image per language

[lucpretti]

When a page is translated into a different language, it's possible to edit all of its settings except the og:image attribute. This is currently managed through the Sidebar option in the Page menu. The problem is that a new page in another language automatically inherits the image uploaded to the sidebar.

So each page (including each translation) needs to have its own og:image.

It's relevant in cases such as:
https://okfn.org/en/projects/digital-public-infrastructure-for-electoral-processes/
https://okfn.org/es/proyectos/infraestructura-publica-digital-para-procesos-electorales/

Both links above currently have the following og:image:

But clearly the Spanish version should have the following:

Next steps

• Make a clear list of what meta depends on language: image, description, title, etc
• Clean no longer needed SideBarExtension
• Create a new TitleMetaTags extension to implement all the tags that are gonna be dependent on the language
• We will need to migrate all the current images to this one (probably manually)
• Create a new PageMegaTags extension to implement all the tags that are not dependent on the language

[pdelboca]

@lucpretti this ticket will require some work (a couple of days) to properly implement it given that now we are not doing things in a good way.

Context

django-cms provides a couple of tools to this with this scenarios:

• PageExtension: use to add fields that should have the same values for the different language versions of a page - for example, an icon.
• TitleExtension: use to add fields that should have language-specific values for different language versions of a page - for example, keywords.
• ExtensionToolbar: to add a menu item in the django-cms UI.

So, all meta tags that do not change with the language needs to be implemented using PageExtension. Those that do change with the language, using TitleExtension.

More info: https://docs.django-cms.org/en/latest/how_to/extending_page_title.html?highlight=ExtensionToolbar

Current implementation

Currently we are using an old obsolete SideBarExtension(PageExtension) to implement the image tag. This should have been done by cleaning the obsolete extension, and implementing a new one.

[lucpretti]

Thank you for the research @amercader @pdelboca!

I'm ok with your approach and with any changes in obsolete/new plugins. Just wanted to make sure you know this is not a super priority - I'd rather implement #995 before.