[back] feat: add the EntityContext model

[GresilleSiffle]

related to #1852

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Description

This PR replaces the poll.moderation system by EntitContext. A model used to represent a textual localized context related to one or more entities.

Those contexts can be enabled or disabled from the admin interface, and can target entities based on any of their metadata. All EntitContext related to an entity are returned by the views using the RecommendationSerializer. When its attribute unsafe is set to True, all related entities will be marked as unsafe.

Scope of this PR:

• refactor the back end to use EntitContext instead of poll.moderation
• make poll views return EntitContext (other views will be updated in following PRs)
• make the analysis page display all EntityContext created by the association (other pages, like comparison will be updated in following PRs)

Out of scope:

• refactor the comparison page
• refactor the entity lists

to-do

back end - model EntityContext

• create the new models
  • delete the concept of source_url and source_label to simplify the model
  • delete the concept of origin?
• create its admin interface
• add the new translations

back end

• tests
  • test the method poll.entity_has_unsafe_context
  • test the method poll.get_entity_contexts
  • test the method entity_poll_rating.unsafe_recommendation_reasons (update the existing tests)
  • test the new behaviours of the recommendations API

back end serializers

• RecommendationSerializer should include the entity contexts

front end

• display != messages when the context's origin is the contributors or the association

front end - analysis page

• display the entity context
• refresh the text of the entity contexts when the UI language changes

front end - recommendations

• (out of scope) display a link to the context (check the miro brainstorm)

front end - comparison

• (out of scope) display the context of each entity

Checklist

• I added the related issue(s) id in the related issues section (if any)
  • if not, delete the related issues section
• I described my changes and my decisions in the PR description
• I read the development guidelines of the CONTRIBUTING.md
• The tests pass and have been updated if relevant
• The code quality check pass

[GresilleSiffle]

Previously this method was always returning True.

[GresilleSiffle]

Let's not forget to delete the field moderation in the next update.