Feat : get proposals from application

[Stef-Rousset]

🎩 Description

This PR updates the module so that the data displayed by the endpoints comes from the application's proposals.
It allows to display external proposals in a participatory space by providing the url of the targeted platform.
It also defines a service to retrieve the proposals from the targeted platform, and displays the external proposals in the proposals index of the participatory space.
Eventually, it defines an external_proposal view to show one external_proposal with its comments.

Testing
See Readme to get and test the different endpoints (for example http://localhost:3000/api/v1/data)

To display external proposals:

1. As an admin, go to a process with proposals, and go to the configuration page of the proposal's component.
2. Check the checkbox "Add an integration", provide the url http://localhost:3000, choose a language and then save.
3. In the FO, go to the proposals index of the process, and see that external proposals are displayed in a card visually different from proposals (screenshot one)
4. Click on one external proposal card and see that the external proposal is displayed with its comments (screenshot two)

Screenshots
ONE

TWO

Tasks

• Update specs