As part of the work to allow multiple users to translate the same topic (T86151), we need to update the Content translation data model to support it.

As the result of this change:

• A user will be able to start a translation for a page, even if some other users have in-progress translations for that page and the same languages. For example, both Cronopio and Fama can start translating "Cupcake" from English to Spanish.
• A user won't be able to have more than one in-progress translation for the same topic and languages. For example, if Cronopio starts a translation for Cupcake from English to Spanish and later tries to start a new translation for such article again, the result will be to continue the earlier in-progress translation (instead of starting a new one).
• A user will be able to start a new translation again even if the user previously published that same translation (this scenario is covered in T192065). For example, if Cronopio published a translation for Cupcake from English to Spanish, and tries to start the same translation again, a new translation will be started from scratch. Note that it should still be possible to continue the existing translation by clicking the pencil from the "published" translations list.