User story

Is your feature request related to a problem? Please describe.
We are adding new options for the Decidim Conversations, such as being able to send messages to groups. This requires a change in the user interface, and in the decidim-design repository there is an interface design that solves this need.

Describe the solution you'd like
We want to use the existing design in decidim-design (https://decidim-design.herokuapp.com/public/profile/user-profile#conversations), since it incorporates improvement elements for the new Conversations options (multi-user conversations and conversations with a group).

Describe alternatives you've considered
Managing the different conversations through mailboxes, but that poses other problems that we cannot address right now.

Additional context
See discussion on this in the issue #5776 (comment)

https://user-images.githubusercontent.com/468685/77765398-80d5cd00-703e-11ea-905d-a1494b85987b.png

Acceptance criteria

• As a participant I see the updated conversation interface with the new design

Related issues

#5681, #5776