ref: ED020

User story

Is your feature request related to a problem? Please describe.
As a participant, I want to be able to have private conversations with more than one person.

Describe the solution you'd like

• I want to have a "New conversation" button and a search engine for participants
• If I select participants with whom a conversation already exists I want to scroll automatically to the conversation
• If I select a participant that has message reception disabled (Add option to "Receive messages from everyone" on my profile  #5679) I want to see her disabled avatar with a message that says "This participant has disabled message reception"

The title of the conversation would be "Conversation with Participant 1, Participant 2 and Participant 3" using their names. It is also necessary to review the title design because the current font size is too big. (Design required)

To stop possible missuses for this feature, it's necessary to have a limit of 10 participants that can be on a given conversation.

Additional context

🎨 Frontend

Does this issue could impact on users private data?
No

Acceptance criteria

• As a participant I have a "New conversation" button in the Conversations page and a users search engine
• As a participant I can search for any participant and select any that has not disabled the option to receive messages
• As a participant I can search for any participant. If the participant has the option to receive messages disabled then she can not be messaged
• As a participant I can select more than one user as the recipient of my message
• As a particpant I can unselect more than one user as the recipient of my message
• As a participant I can not add more than 9 participants to the conversation.