Feat/add multiple platforms

[Stef-Rousset]

🎩 Description

This PR updates the module so that it is possible to integrate multiple platforms.

Testing

1. As a user go to the index of proposals for a process, and note the number of proposals displayed, here it is 5 (screenshot one)
2. As an admin, go to that process > go to the configuration page of the proposal's component.
3. Check the checkbox "Add an integration", provide the url http://localhost:3000, choose a language and then save.
4. In the FO, go to the proposals index of the process, and note the total of proposals, like her 29 proposals, so there is 24 external_proposals (screenshot two)
5. Go back to the BO, and add a second platform to integrate, like this "http://localhost:3000, http://localhost:3000", and save.
6. Go back in the FO to the index of proposals, and check that the number of available proposals is equal to 5 + (24x2) = 53 proposals (screenshot three)
7. Click on one external proposal card and see that the external proposal is displayed with its comments

Screenshots
ONE

TWO

THREE