Fix proposal creation

[mrcasals]

🎩 What? Why?

After #6285 and #6384, some bugs were still found:

• In some cases, proposal title and body were persisted as simple strings, instead of JSONB content in the DB.
• Proposals couldn't be created as i18n from the admin side.

This PR solves both issues.

📌 Related Issues

• Related to Make proposals handle i18n #6285, Fix bugs after proposals i18n move #6384

📋 Subtasks

None

📷 Screenshots (optional)

None

[mrcasals]

Hi @decidim/product! When we moved proposals to i18n back in #6285 (and related #6384) we introduced a few bugs, this PR solves them. But also, this PR adds the ability for admin users to create i18n proposals from the admin site. This means official proposals can be translated! And only official proposals: we've skipped participatory texts, and non-admin users can only create proposals in their current language. We think this is more consistent to the rest of the admin.

We've opened a review app here: https://decidim-staging-pr-169.herokuapp.com/ can you check it out, please?

[carolromero]

@mrcasals I checked this, working as expected! I just found that there are new validation rules that step on each other, but I don't know if this has been introduced by this PR:

[mrcasals]

@carolromero I suspect we always had this problem, but never realized it. I went back to develop and tried it out (notice how the fields are not i18n):

Can we open an issue to keep track of that problem?

[carolromero]

@mrcasals must be recent because in release 0.22 it does not yet appear, my bet it has come up with some of the accessibility changes

Can we open an issue to keep track of that problem?

Yes, we can! Right away 😄

[mrcasals]

@carolromero Thanks!!

@decidim/core can you review this, please? 😄

[ivan-mr]

Great job @mrcasals. Thanks!