Replace TimelineMessage aggregate by Subscription.NotifyFollower

[devcrafting]

Pros :

• TimelineMessage is a pseudo-aggregate, that take no decisions
• TimelineMessagePublished do the same than NotifyFollower and FolloweeMessagePublished (could be renamed FolloweeMessageReceived perhaps, it would reflect more that it is related to reception on Timeline)
• FolloweeMessagePublished/Received better describe why the message is added to Timeline, instead of TimelineMessagePublished technical event

Cons :

• a little bit more inter-dependencies between Message and Subscription sub-domains, but extracting a Timeline sub-domain (with Handler and TimelineMessageProjection only, i.e no aggregate) would reduce them