Add activity history to proposal

[loicknuchel]

Proposal is the only entity that is shared among different kind of users (orgas & speakers) and it's a central entity to collaborate between them.
So it can be nice to build an activity stream for it so orgas & speakers can easily understand what is going on.

Events to track and display:

• Creation: "Proposed by {{speaker.name}} to {{cfp.name}} on {{datetime}}" or "Requested by {{orga.name}} for {{cfp.name}} on {{datetime}}"
• Edition: "Edited by {{user.name}} on {{datetime}}" (with eventually modified fields)
• Accepted/Rejected
• Comment added

Can be done using event sourcing, eventually... To be discussed

We can take inspiration from github issues/PR ;)