Allow administrators to open a conversation with the author of an initiative

**Is your feature request related to a problem? Please describe.** When an author sends to technical validation an initiative the administrator might need to contact him/her firs in order to validate the petition (validate a change, ask for details etc.). Currently the only way to do this is using conversation feature but the message lacks the context of the petition. **Describe the solution you'd like** The best would be to be able to have a conversation on the petition itself a little bit like like it happens with the private notes and proposal. It's really useful to have the proposal content above. This would allow to receive specific notification such as "You've received a message about your [Petition title]". Co-authors would need to receive these notifications too. **Describe alternatives you've considered** Using the existing conversation feature. **Additional context** No **Does this issue could impact on users private data?** No