Snippet translations + translatable fields

[KalobTaulien]

Closes #6708
Related PRs/issues #6703

Acceptance criteria

Note: Don't forget to reference the document from Step 5 #6707

• Create migrations for all snippets. This document will be very helpful. We need to bootstrap migrations for each model.
  • Make sure all snippet models are translated.
  • Make sure all snippet fields have translatable_fields = [ .. ] set on them
  • Make sure tests still pass. May need to do some adjusting for new language headers in the request object, particularly around redirects.
• Get Theo to review this (specifically, Theo) to make sure all the fields are the right translation type.

[KalobTaulien]

Locally all the tests pass.

@Pomax I pinged you for code review on this.
@TheoChevalier I pinged you for semi-code review but mostly to make sure I got all the fields right. Your doc was very helpful. I made some notes in the doc too and highlighted some lines for you to take a look at.

[Pomax]

some minor notes, and a question

[Pomax]

Let's check if this is actually necessary or not. If we don't need to mark the Meta class as translatable, that saves some migration work. (not much, but "not doing anything we don't need to do is worth not doing" =D)

[TheoChevalier]

I’ve made a few changes to the spreadsheet (all highlighted in orange), mostly answering your questions.
A few fields in snippets are not properly hooked (I’ve highlighted them in red in the Snippets sheet)

Also hit a few errors/have some questions:

PNI General product:

Translating a general product raises:

Campaign page

Translating a campaign page that has a petition + donate snippets raises:

Youtube Regrets

Regret stories are missing. Is this because it’s a block and not part of the model?

Article pages:

For Text and ID https://docs.google.com/spreadsheets/d/1AU7pxboZoBiVcbTpZO-VUMAhxZwx5mdEH_FBLfo2D-w/edit#gid=0&range=A43 these are fields I’ve found at the bottom of article pages, but I haven’t been able to find them in the model code

Homepage

Here too, a lot of the content is not localized, is this because these are blocks and it will come later?
Also it seems every page picker field need to be Translatable, otherwise links will point to /en/…
This affects all page pickers in News you can use, Spotlight, Take action, etc

[Pomax]

looks like snippets at least work now, but we'll need to comb the code on Monday to make sure we caught all of them. We'll also want to fix what looks like a migration numbering issue.

[Pomax]

Let's get this landed and then deal with the followup as new work.

[KalobTaulien]

Ticket made for the campaign page problem. #6851